If his best attribute is being a lock-down defender, his opportunities for rebounds will be fewer as a natural byproduct of his assigned role, since he will constantly be on-ball and contesting the other team's best shooters. He'll frequently find himself contesting threes away from the rim. But sure, he can probably do a little bit better in that category. I won't argue that. Although, Ali Khalifa wasn't much of a rebounder, either. Yet he changed the game by what he brought to the table.
If BYU had Mag last year, BYU would have won games like @TT where BYU had Pop Isaacs go bananas from the 3-point line. His rebounding is a distant concern comparatively.
He's a role player, and that's fine. He'll do a good job when called on for specific assignments. That's the primary reason he got an offer.