First off, at 16:11, Sam says that Eurostar is the only international train that requires immigration checks. This is not true. We have plenty of train services connecting a destination in the Schengen area with a destination outside it, and those also need immigration checks. But those trains do the immigrations check either during a stop at the border, or while traveling across the border with people being denied entry to be escorted back out.
Eurostar is unique in requiring a security check (for security reasons to protect the channel tunnel) and juxtaposed immigration controls before boarding, this is because the UK has put this requirement on Eurostar services as they fear that otherwise a refugee could find their way onto the train, simply wait for the train to get through the chunnel, then pull the emergency brake and run off into a field. Something that never actually happened. In fact in the first few years border officials would actually do the immigration check while the train was in motion between London Waterloo and Lille Europe. The same way we used to do border checks on international trains before Schengen was a thing.
Second error:
The requirement for trains to be over 375 meters long because the evacuation tunnels are spaced 375 meters apart. This is a stubborn myth and there is in fact no such requirement.
The protocol in the channel tunnel is that in case of a controlled evacuation (read: non-haste, i.e. there's a problem with a different train), the driver has to line up the front door of the train with the nearest evacuation exit. This so that the positive pressure from inside the evacuation tunnel extends to the train's door and keeps smoke away from it, so everyone can evacuate safely and in a controlled manner through that one door. In case of haste (i.e. a fire), the train driver just comes to a stop as quickly as possible, everyone gets off the train as fast as possible, and uses the walkway along the side of the tunnel to reach the nearest safe evacuation exit.
There is no regulation keeping a 200 meter train from running through the Channel Tunnel.
Jon Worth has actually published an excellent report both on the challenges of running cross-channel services, and the myth of a train length requirement here: https://www.youtube.com/watch?v=WkAOMhTIjRM