My DF’s Parkinson’s manifested itself just like this. The debate took place in the evening when Biden might have been sundowning. His very quiet speech, tailing off at the end of a sentence, starting coherently but losing his thread and petering out into silence, all very recognisable Parkinson traits.
The following day he addressed a rally where he was absolutely present, coherent, audible and articulate. Again, very recognisable.
The question American voters have to ask is whether they would prefer a President with failing health who makes sense most of the time but will need a good support system, or one who shouts and blusters but makes no sense at all.