The mindset is a little narrow on this topic.
When we say 8-bit music, we automatically think about music produced by an 8-bit video game console (because that's what's meant most of the time). There are however other 8-bit musical instruments not intended for video games. You could call music made with those instruments also 8-bit music.
Btw, did you know that even though the snes was a 16-bit console, the audio cpu had an 8-bit core? So you could call snes music 8-bit as well.
Not trying to make it overly complicated. I think it's relatively clear what's allowed in this competiton. Just saying.
Ps, I'm working on some songs right now of which I want to use one for the competition. But I'm not sure if they qualify. Some remind of 8-bit songs, and are in a sense 8-bit. Others are made with 8-bit synths but don't sound like "gameboy music".