In Celeste, there's an easter egg that lets you play the original PICO-8 game on an in-game computer. How was this accomplished, if porting it themselves isn't possible? Did they have to remake the entire game?
They might have built their own pico-8 runtime/emulator. Alternatively, porting the game to use another Lua runtime (like LÖVE) would probably not be that hard. Ex, here's a pico-8 runtime in LÖVE: