Biden is not capable of forming a coherent, cogent and intelligible thought when it is to be done without any aid other than his pudding brain.
Having been around dementia patients, they do have “good” days where they are more or less back to their old selves. The issue is as things degrade those come less and less often and then no longer at all.
I would say Joe in my opinion is probably ok half the time, so it was a 50-50 chance they took putting him on that stage. In fact if the debate had been an hour later, he might have gotten away with it.