It's a partisan game. The people who rejoice in every Trumpism (as did some over Bushisms), and solemnly pretend to worry whether it means the president's mind is slipping have no interest in whether Biden's verbal tics, occasionally confused and mangled utterances, and apparent weariness mean anything; the people who wonder about Biden have not much interest in finding fault with Trump (nor with Bush, in his time).
Just as there are Republicans who will not vote for Trump, choosing either Biden or a third candidate, there are Democrats whose first choice was not Biden, who would welcome an excuse to replace him. The progressive wing in particular would prefer someone more progressive. They can't have Bernie, but Kamala will do, especially since she is not also old-white-man. But first Biden has to win, so the conversation isn't loud. If some people are casually and openly spitballing competency and corruption what-ifs, it is easy to imagine that some are thinking the possibilities through seriously and in detail.
