I don't think he's a third down back, which is why he is probably rated that high.
He's a three-down back, who happens to catch exceedingly well. So on a team with an established RB, he could certainly function as a third-down back, due to his superior receiving skills.
But he's a tougher runner than some give him credit for, as evidenced by numerous plays he made this year.
I 100% acknowledge that no NFL prospect is a sure thing, and maybe I'm wrong. But I think that rating is based upon him being such an exceptional receiver ON TOP OF being a tough runner.