Actually, the sprites were made in Aseprite, it's what I use to make pixel art, and there's the PICO-8 palette pre-installed in it, so all the sprites are made in Aseprite, using the PICO-8 palette, and exported the sprites
Oh thanks for clearing that up :) I use Aseprite but I didn't notice PICO-8 was one of the available palettes, it's good to know. I assume it's to be able to paint in Aseprite to then import the graphics in PICO.