“Games with music will be muted in the showcase livestream if: you didn’t make the music yourself, the music is AI generated, or you plan to copyright it.”

I suppose I can understand the last 2 rules.

But all music has to be original or it gets muted? Even CC0 or assets we own?

Correct!

Whenever I've allowed non-original game music to play during streams in the past (such as CC0/owned assets), some of the tracks end up getting content claimed on YouTube. Since there's no way to guarantee whether someone's track will set this off, this rule was implemented.