There was a time when brain dead Biden, (2012 - 12 years ago), could debate.He is no longer that person. President Trump remembers that and made the comment about not underestimating Biden because of what Biden was at one time able to do. From the article you allude to -I was never a fan of his, but I will say he beat Paul Ryan — still years ago, but he beat Paul Ryan badly,” Trump said. “And I assume he’s going to be somebody that will be a worthy debater, I would say, I think. I don’t want to underestimate him.”- I am sure Biden will be well prepped and the soft ball questions he will be asked will be structured where he should be able to answer with just a few words. I don't think that President Trump is overly concerned with Biden, per say, but remains cautious because of the puppet masters who are controlling Biden and what they will allow Biden to say.