Celeste is a fantastic game I am still yet to finish. But I do have to give kudos where it's due, it is so well done, so polished, the movement feels great, the story is outstanding, the graphics are beautiful. The soundtrack by Lena Raine? Amazing. Do give it a try.

Obviously the link is primarily talking about the fact that the player function is 4000 lines long. That's a lot. Maybe that's the lesson, your code does not have to be perfect, the result must.

I'll also add to the discussion this amazing video by GMTK, which explains what video game developers must think about when designing movement. It's wonderful.

Why Does Celeste Feel So Good to Play? (https://www.youtube.com/watch?v=yorTG9at90g)