As I pointed out before, it's smart to play up the debating ability of your foe. Because if they then fall short, you declare victory. Minimizing your opponent's abilities is foolish because all they have to do is exceed the very low bar you set. But Trump is too undisciplined and too much of a bully to follow that truism.
And finally, what do you know: Trump campaign plays up Biden's skills ahead of Cleveland debate: 'He's actually quite good' (The Hill)
If Trump (and some of the dim bulb supporters) had listened to me, he'd be in much better shape!