He says he believes in compatibilist free will, where all our actions are determined by past events, but we have an illusion of making choices.
Here is a recent philosophy paper on free will. It also defends free will only in some contrived sense.
If free will violates the laws of physics, then what law is violated? Where is the scientific paper that made this discovery? Who got the Nobel Prize for this scientific breakthrough that resolved millennia of philosophical arguments?
None of this can be explained, of course. Carroll is just relying on his peculiar prejudices.
He has a few, if you listen to him. The biggest is that he subscribes to many-worlds theory. That really is contrary to a scientific understanding of the world. Just listen to him try to explain how he might be split into an identical copy who is then wiped out by a vacuum decay in a parallel world. And how probabilities have no meaning in many-worlds, but we try to be good Bayesians anyway, and probability is how we like to think of the world. It is all the same as if he lives in an imaginary simulation where anything can happen.