Well, there actually is/was a Supertux 2. I don't remember the specifics, I think they rewrote it from blit 2D into OpenGL? You can see here : https://github.com/SuperTux/supertux/blob/master/supertux2.d... that it's called supertux2

But yeah this has nothing to do with the actual content of the levels, it only refers to the engine.