I think the credit for Josh Allen's improvement is mostly on himself, he put the work in on the offseason with Jordan Palmer and got better each year.
The coaching continuity helped I am sure, but I am sure Josh Allen would have still gotten better if on the Jets, but definitely not putting up these kind of numbers as the supporting cast is not good.
Josh didn't have a lot of reps in college, some say Baker/Darnold already had 10,000 more throwing reps then Allen, so Allen was always about potential. Darnold was way closer to his ceiling.