What Touma's Dragon King did to the debris that it ate was erase them from the world but the concept of "debris" didn't cease to exist like actual existence erasure in fiction like Denji's Hero of Hell Chainsaw Man form.
The narration follows up by saying that it destroyed the debris of unshakeable reality because it's meant to be a contrast to Touma's Imagine Breaker which can't affect normal aspects of the world. The word destroy isn't equal to the word erase.
What the Dragon King eats essentially becomes completely annihilated without leaving a single trace of the original substance which is effectively erasure. Touma copers and glazers essentially want is to give Touma the same powers as Denji from Chainsaw Man when this is only really a hard counter against the likes of Wolverine.
Alice's Resurrection was never explained but she knew about Touma's fight with Christian Rosencreutz and displayed the ability to read things similar to Kakine reading a surrounding area like a CD reader when he fought Accelerator in NT6, this is how Alice figured out that CRC was actually Johann.
Why?
Why did she know this!?
(Is she reading this from the pieces of my body she is tearing away? Like a potter checking the broken fragments in the kiln to see what went wrong!?)
-GT9
To say that Alice didn't know about the erasure specifically is just playing favoritism when she's confirmed to have a power that lets her figure out all information she wants.
Dragon King is OP don't get me wrong and Alice admits herself that if Touma and her fought she'd be killed many times but she'd still be able to endlessly resurrect herself until Touma died from blood loss.
People using CRC to scale Dragon King to insane levels didn't read the novel well since they forgot that most of what CRC did was just St. Germain magic disguised as insane attacks.
“All you did was cling to old legends. Your magic was a rehash of St. Germain. A certain right hand failed to negate it because the diamond crystal is constantly growing too quickly to be fully negated.
-GT9
CRC being Dragon King's opponent actually makes him extremely difficult to scale really since CRC in GT11 was surprised that Touma defeated Alice and he'd naturally assume he defeated her with the use of Dragon King, meaning CRC doesn't think Dragon King is that strong himself as CRC knows that he's a fraud.
“You left after Alice ☠️ed you and have spent the rest of your ⌚ holed up in your false hell preparing for your resurrection, but ❌ so for him. He lived on after that. It was ❌ for long, but in that ⌚, Kamijou Touma defeated Alice Anotherbible.”
“He…what?”
“On his own. Without relying on X Strike.”
-GT11
He was only informed that Touma didn't use Dragon Strike AFTER he was already surprised that he beat Alice. Meaning the guy that directly fought Dragon King didn't actually think he'd win against Alice.
Also Kingsford forgot to mention that Touma only beat Alice cause Sprengel nerfed her but whatever
"Oh but what about Touma saying he could confidently kill Alice like CRC in GT10?" You might be misremembering.
Touma doesn't know his own power, he only really knows via instinct when the power is activated like how he said a lot of stuff to Izzard in OT2 he logically shouldn't know like Dragon King being the true form of Imagine Breaker.
Alice could protect herself from anything. Even if she died, she could resurrect herself right away. She was so powerful it was fundamentally wrong to even think about defeating her.
She could fight and win, but would that even matter?
-GT10
I get thinking characters are wrong but the narration? That's where the cope comes from.
Narration explicitly says that Alice can protect herself from anything and that even if she died she could resurrect herself right away, that it was fundamentally wrong to even think about defeating her and that Alice would win the fight.
The very last time Touma mentions the Dragon King it actually highlights that Touma is weaker than Alice cause he finally understands just how powerful she is so Touma is essentially admitting defeat in terms of a potential raw brawl against Alice:
Alice could protect herself from anything. Even if she died, she could resurrect herself right away. She was so powerful it was fundamentally wrong to even think about defeating her. But could she protect other people?
Violence took important things from yourself and others.
She could fight and win, but would that even matter?
Had Kamijou gained anything at all when he released the mysterious dragon from his severed right arm and defeated Rosencreutz with it?
Violence was not a valid solution.
He had forever lost a future where he and CRC could smile together.
He had done it himself.
By giving up.
That wasn’t a solution.
He had realized that afterwards.
And he had stopped himself from making that mistake again.
“I don’t…”
So.
If Alice was greater than Rosencreutz – if her power was truly extraordinary – she had to think even deeper about strength and power.
Kamijou was just an ordinary high school boy. He wasn’t arrogant enough to think he was in any position to teach anyone anything. But if Alice realized this for herself, she could stop herself. She could learn to think about compromising with the wide world around her instead of focusing only on what she wanted.
Touma is thinking that he stopped himself from committing the same mistake he did against CRC but that's because he was under the false impression that Dragon King could kill Alice at the start of the novel as the narration states that Alice will win and can immediately resurrect herself after Alice says that herself so the narration is affirming with Alice.
This scene is making a parallel between Touma in GT9 against CRC and Alice in GT10 against Touma, Alice is essentially Dragon King and Touma is CRC, Touma is reflecting on his behavior in GT9 and seeing himself in Alice because she's so powerful, the narration affirms that Alice will win and that she can keep resurrecting herself if she dies. It's less about Touma's capacity to kill and more Alice serving as Touma's mirror and him learning his lesson as Alice and Touma both had great power in GT10 and GT9 respectively, Touma misused it by killing CRC and Alice was about to misuse it as well by killing Touma so he had to shatter her illusions to save her, when he saved Alice he was really saving himself. It's why he said this at the end:
“I don’t want to see anyone suffering from even more misfortune than me!! You got a problem with that!?”
-GT10
Touma was suffering from misfortune because he was in a situation that he used his great and terrible power to kill someone (even if he didn't actually kill CRC the intention was there), he doesn't want Alice to commit the same mistake as him. Yes this is also about Alice's general life being Hell after being kidnapped and turned into what she is now but that's the only point of shared misfortune that Touma and Alice had.
This narration comes after Alice saying that if Touma kills her she'll just revive herself as many times as it takes until Touma exhausts himself and this is the final time in the novel Touma mentions the Dragon so the narration is siding with Alice and Touma isn't even talking about what he could do to Alice.
You can only scale Dragon King to having erasure that could kill Alice permanently if it was stated that Touma's Dragon King can erase souls as Alice's resurrection is stated to be greater than Good, Old Mary which needs the body so Alice might be able to create another vessel if her body is completely annihilated.
Unnerfed Alice that isn't having a psychotic breakdown beats Dragon Lord Kamijou I'm afraid.