I think that the comparison to an iPhone Pro Max is unfair. First, it runs a different OS with different apps. Second, macOS behaves differently and has to deal with a completely different window management. I compared scrolling between my 24" iMac and my new 14 MBP, and the difference is big. Everything is much smoother on my MBP, especially when scrolling slowly. You can feel the responsiveness and smoothness. Also, OS animations are much better than before. Having said that, of course going back to 60Hz is not bad. I used my iMac for a while and I found scrolling to be ok, but not as good as with ProMotion of course.
Anyway, I don't see any stutter and I love the display.