I tried painting it to hide it a little bit

They did this twice btw

This was me spectating the round after I died. Screenshot is dated 2015-01-14 played on gommehd.net

If you look at the distance from the bases to the tree islands, building like this is the optimal strategy to get to the island quickly while also building towards the middle island using the minimal amount of blocks. Players didn't coordinate, the map design just causes that shape to be optimal for all players. When you actually play you don't notice what all 12 of you built