The Extremely OK Games team, on the project’s GitHub, writes (minor grammatical edit by me):

“Celeste 64: Fragments of the Mountain [is] a game made by the original Celeste developers in under 2 weeks for Celeste’s 6th Anniversary. We haven’t done a lot of 3D development so much of this is not very optimized, it’s coded in libraries mostly intended for 2D games, and we put it all together very, very quickly. Consider this similar to a weekend jam game, so experiment and learn at your own risk!”

A prebuilt version of the game is available for Windows on itch.io for free, too!