Just caught up on the VOD of Celeste. The PICO-8 version you found at the end of the stream is the actual original version of the game. You can still find the original PICO-8 cartridge live and on-line here: https://www.lexaloffle.com/bbs/?tid=2145. Looks like a few people in that thread made mods for it which is pretty cool. The PICO-8 version was also ran at SGDQ2016 here: