There's a common misconception here that derma rolling is mainly for collagen, which is also stated in the FAQ, but that's not correct if we're worried about growing hairs like you pointed out here.

Actually, you don't need collagen at all to grow new hairs, and this is simply seen in aging men. If collagen were a requirement for hair growth, why do men grow more facial hair as they age and lose more collagen?

Why can very old people, ie people 60+, still grow thick, dense beards despite having less than half the collagen on their faces from when they were in their primes?

The main point of derma rolling for hair growth isn't collagen, but the growth factors that can create new hair follicles and continue to grow existing ones.