He's saying that when he calls the cbet, he faces a pretty wide villain range (anything the villain might cbet here, including a lot of bluffs), and AA do great against that range.
If he raises and the villain reraises, then his AA would be in pretty bad shape (facing likely QJ, QQ, JJ, 44, at worst a combo draw, which is less likely because he holds A

which blocks out villain's nfd combos).
Does he have a read that this particular villain is solid?