It doesn't make any sense if you ask me.
If there is a really big pot on the turn and/or river what's stopping someone from commenting that they had folded quads/boat/flush and etc.
I mean we all had those garbage
hands that miraculously flops trips/straights/full house/ even quads! However because of poker etiquette, most people don't talk about their hand until after the current hand is over. It would be messed up if someone bets over the pot trying to
bluff a hand for example xxx44 and someone mentions that they had folded 44. They could be lying, but if they don't like the river bettor or they're friendly with the potential caller they can give them critical information.
So if they're going to disable the chat for an all-in they should just go ahead and disable the chat completely. I'd prefer the chat 24/7, even during an allin. Would make for some interesting comments.