while Darnold has his feet still on the ground.

I am purely speculating here, but I would suspect it might be actually easier when you are running perpendicular to your target if there isn't an anchor point on the ground in cases like that. The anchor on the ground would hold your body back affecting your ability to follow through with the pass. On the contrary, if you jump throw in that situation, your lower body turns with your upper body and then it becomes just arm motion guiding the throw.

Zach threw that on a dime. Darnold couldn't get his body around far enough and ended up throwing the pass three or four yards wide to the right.

I'm not sure if that is something that quarterback coaches would routinely teach or if that is just something that was instinctual for Zach and that as a result of it he has worked on it.

That is a pass that in the entire history of football was probably coached to never be thrown. Zach Wilson would have probably given football coaches even 10 years ago coronaries. Yet suddenly off platform throws are all the rage. And Zach is very good at them.