Some speculation

 I agree with this:

and I hope he does.  I reckon all Democrats should be saying repeatedly that it is nuts and any politician expressly any sympathy to such a dangerous fantasy of violent revenge against imaginary crimes is a danger to the nation.  

But - I bet if Biden did put out this challenge, Trump would respond with some mealy mouthed "I don't know.  I don't know if any of it is true or not; but who knows? There might be an element of truth in there somewhere.  And I see more danger from rioting in the streets by anarchists." And his cult followers would praise him for being an open minded and fair President who has his priorities right.