I've never said he can't, but he doesn't. I know back in the day that many young QBs struggled with the footwork. That's why QBs would get a boost in draft status if they ran a, "pro style," offense. I doubt Moore ever did it at Boise State, for example. People underestimate how much it helps the running game being able to throw from under center. It keeps the defense honest and can create 1 on 1s or other mismatches in the passing game if done correctly.
Walsh said what made Montana great was his footwork. Said he was like a ballerina. Those drops are designed to work with certain route trees so that the ball comes out after that back foot is planted. When done right it's a thing of beauty.