The thing about Celeste is that it's essentially two games in one. The first game is when you play it with only the abilities explicitly taught by the game up to that point (for most of the game, just jump, climb, and dash), and the second is when you go back and replay it from the beginning, having learned about wallbounces and hyper dashes and such. The level layouts of the "second game" are all identical to the first, of course, but the traversal becomes completely different with the mobility allowed by these newly-learned moves. (More recently, you could argue a third game has developed around un-taught and unintended tech like ultra dashes and demodashes, but you get the idea.)

But at least for me, the existence of this second game (and third) does not in any way detract from the quality or enjoyability of the first one. The speedrunners are basically playing a different game, and unlike most other video games, in Celeste this is by design.