The helmet it kind of sort of looks like he’s hitting is actually a players about five yards away. Bit of optical illusion there. The player he’s actually tackling has the helmet above his with about 6 inches of clearance, Harrison hit the QB right in the sternum but without a spearing action. Probably hurt like hell, and I don’t doubt the QB spent a couple seconds relearning how to breath after, but it’s a legal hit.
It looked to me like he was hitting the helmet/facemask of #14, but like I said, the angle is such that it’s hard to see whether the helmet was going into the shoulder pad or helmet of #14.
I suppose the moral of the story for Harrison is not to lead with his helmet.