Only one rational conclusion to be drawn.
Trump thinks Mueller makes him look like a victim, and that this somehow rebounds to his political benefit in 2020.
I don’t like it, but apparently that’s what Trump thinks.
In this entire country Trump couldn’t find someone who isn’t a close friend of Mueller for AG? WTH?
My immediate thought was that Trump is not afraid of what will be revealed. Perhaps for the reason you mention but nevertheless, not threatening to him. I trust that as POTUS, he probably has some pretty solid information about what is going on in the investigation. If Mueller comes up with nothing but trivialities, only CNN and MSNBC will get worked up about it. The general public will wonder why the Washington Establishment wasted some much time and effort on a nothing burger.