Well the fucked part is that he's already lied about Hunter plenty of times before. Look at how he behaved during the last debate. When Trump brought up that Hunter was ejected from the Navy because he's a meth addict who flunked every drug test he was given, Biden started shaking his head and repeatedly mumbling, "That's not true. It's not true." even though it's an objectively true statement.
He has no problems lying about Hunter at all, but this time he's locked himself in a basement and by the end of this stretch of lids he'll have been avoiding this topic for 8 days straight. That looks really fucking weird when you're smack in the middle of the hottest point of an election. An accusation like that ought to have the candidate flying out of his corner with his fists spinning like windmills, but it made Biden whimper, duck into a corner and start trying to dig his way through the mat to hide under the stage.
That's not a good response.