I'll say, I garnered that he wants to put his qbs is situations to fail in training camp so he can see how they respond. Imo thus is a way to grow him with no pressure. If he beats himself up "whats the issue, its OTAs", if he cares too much "stop being hard on yourself, its OTAs".
Actually, if you check out Darnold's highlight reels, you'll find he's perfectly capable of running.
(given how bad the Jets were, he got plenty of practice)
He's just not necessarily somebody you call a bunch of designed runs for.