.
Maybe he was just attempting a half-baked gesture of unity, but I'm speculating that even Joe Biden is not THAT stupid.
That Joe Biden hates Kamala Harris for usurping him, and that putting on a Trump hat was Biden's way of sticking it to Kamala Harris and the Democrat party elites for rejecting him.
And that Biden, if Kamala Harris loses, will follow it up with an "I-told-you-so" book, browbeating the Democrat party for selecting Kamala over him, and how they could have won if they'd just stuck with him.