You asked "how they could know" as if there aren't hours of tape of him at LT. That's how they know. Could he move? Sure. Was just trying to simplify things for you.
Do you understand your last sentence is insulting? It is a poor way to engage in an even-handed discussion.
Coaches continually state that you never know about a player until he gets on campus. It's a coaching mantra.
Despite all the film study, despite scouting in person, how can it be that a number of student-athletes never reach the heights coaches believed they could reach?
Maybe others here would like to compose such a list. Please give it a go with a few prominent names.
Then we'll ask this guy why the coaches didn't know the kids would fail but nevertheless offered them scholarships.