Bandcamp

I wouldn’t overthink it. When they say ‘substantially’ they’re basically just banning obvious slop. If you make something people genuinely enjoy you’re not likely to get policed if the track has a little AI assist somewhere in the pipeline

What stops me from asking an LLM "Give me 5 chord progressions and scales that sound like Adele" and glue it together? How could it be detected?

There is in fact nothing stopping you from simply using the exact same chord progressions as her. No AI is needed for this task. You could literally take her sheet music, white out the lyrics and tune, and build your new song directly on top of the existing structure if you wanted. Chords and scales are not protected in any way :p