If you slow the video down, he hits him in a way to come up under his chin strap. The angle he hit him wasn't meant to be "on the numbers", it was meant to drive into his helmet.
If you pause it, his helmet is basically making contact with his chin.
Most of us would probably have been knocked out. And the reason that's illegal, is because any defender could accidentally kill guys if they allowed defenders to come under chinstraps. Imagine Bobby Wagner coming under Dez' chinstrap immediately after a catch, it'd make the Lockett hit look like peewee football. Probably end Dez' career.
If the Cowboys were a dirty team and this was the 70's, I don't think Tate would make it out of this game alive. That's why we have rules.
I'd love to see Lee blindside Tate's 5'10 body like that, except in that case Tate's head might physically leave his body. Tate wants to act cocky, if a 235 pound guy did that to him, there'd be serious concern that he ever actually plays again. See Lockett, and that was a clean hit.