Humans generally are not reasoned into beliefs, but use reason to justify their predispositions. And then they feel as though they have gained their beliefs through reason.
This means that I in fact can predict a person's political dispositions toward Trump retroactively from their response to Biden's speech with a high degree of accuracy.
That is, if we had set up a wager on whether I could retroactively predict the pro/anti Trump dispositions of some random set of people based on their responses to Biden's speech, I would have won far more than I lost.
And for the record, nothing made me despair more as a political scientist than learning this truth. Logic is largely irrelevant to politics and to policy as well. If you want to persuade, play on people's emotions. You've seen it happen over and over, from Reagan to Clinton to Obama to Trump. If you don't believe it yet, you, too, may be impervious to logic.