I'm only one of the playtesters, not the developer, so don't quote me on this, but I believe it refers to the lighting/shading/rendering engine sixe created (I think), which plugs into gamemaker. One of the features on the steam page says "A novel pixel art style, rendered by the atmospheric PIXEL2.0 engine." For example if you do a google search for "gamemaker rendering engine" you'll find various results where people have created 3D engines, shading/lighting engines, etc for gamemaker - it's like that. There's also some points in the game with pseudo-3D effects (like the moth in the screenshots here on itch, it looks really damn good in motion) - I'm not sure if that's considered to be part of it though.