I don't think it has to do with anything regarding philosophy on QB athleticism. I think it has everything to do with not being able to impact that aspect of a player as a coach. You can't coach physical traits. All you can do is prepare a guy to play the best he can with what he has to work with.
Darnold isn't a a statue but he isn't a big run threat. He has a big arm but he has a pretty big wind-up as well. If he has sufficient grasp of the offense, might as well throw the kitchen sink at him and try to figure out if he's gonna sink or swim before the live bullets start to fly. See if you can open up the playbook or if you're going to be forced to dial it back.
Good for Rhule!
Sam's issue isn't physical. He can run and make all the throws.
Darnold's weakness is making good decisions in a timely fashion.
Forcing Sam to interpret defenses and make quicker decisions is the only way he is going to improve and ultimately be successful in this league.