And Arsenal have had Saka, ESR, Nelson, Nketiah, Nwaneri, and MLS all within the last ~5 years (obviously varying degrees of playing time). And we have a pretty rich history/culture of using our youth players under Wenger before that.
Amad and Garnacho weren't even internally developed youth players. Man United bought them at 16/18. I guess that's irrelevant for Heaven, because that's the same route for him, but Man United doesn't have any better track record than Arsenal does. I'd argue it's worse if anything.