As a semi-professional musician, sounds fair to me.

You wrote the lyrics. There are professional songwriters with many hit songs who only write lyrics. Some can't even play an instrument, much less compose music. So what do they do? They work with a music composer. They hire a music arranger. They hire a band.

So in this case, you still did the foundational songwriting part yourself, but instead of hiring humans to help you finish it, you hired AI.