Subject to your compliance with our Terms, we assign to you all of our right, title, and interest—if any—in Outputs.

Originally it was not that way. They changed it because, as you suggested, their service has questionable utility without it.

The outstanding one is the “meaningful human input” part of copyright. The most recent ruling is that prompting alone does not count. If you write/rewrite sections, those are yours. Everything in between is a somewhat untested.