To be fair Leinhart was drafted 10th overall. But that was before Whisenhunt got there.
That's true. But like you said, Whisenhunt didn't draft him. Also, I didn't watch Leinart play enough for me to know too much about him, but I'd guess he greatly benefited from the talent at USC during his time there. That probably helped him get drafted at #10, especially since the old CBA was still in place, pushing teams to draft "big" positions, since they didn't want to give a lot of money to "lesser" positions.