I am not sure that I agree, but you clearly articulated your point so I am upvoting nonetheless:-).
For myself, it depends on the facts, which I am not in full possession of. Everyone has bad days imho - that's not "news" at all - but more important is what that performance indicates about his current state. And more importantly, the state he would be in 4 years from now.
This puts the nail in coffin, so to speak, on the idea that he would even live through his Presidency, much less remain salient throughout. A vote for him is a vote for Harris, as the article suggests, and talks about how acknowledging that reality might help pick up more independent voters that could turn the tide of the election.