That they are cutting him off seemingly before he says something monumentally stupid is a pretty clear indication that there is some sort of buffer in the feed, perhaps only a few seconds with a basic "panic button" input to cut the feed at t - X seconds, where X is the length of their buffer, probably 10-15 seconds.

They have to do this. He is a completely senile senior citizen and has no political future at this point. He's 80 pretty soon.

Unreleased comments from bidens cut off mic - "Come on man,Tapioca ... you know the thing "

I'd guess that it had been running for a while and Biden was moving on to something else, thus they ended the video feed.