Do hill repeats make you faster on flats?
Yes.
A hill workout is a speed workout without beating the shit out of your joints. I do them twice a week and only do actual speed sets in the meat of my longer runs for a short time. 5’ of 30":30" tempo pick-ups or a set of 6-10 strides of 60"-2’.
My reasonong is that I’m 43 years old and want to keep my knees working for several more decades and doing a lot of speed workouts will decrease my chances of that being realistic.
Enjoy.