you're right it's a grey area, but in the field of music specifically, it's some of the most stringent requirements. the more creative it is, the more it replaces the market for the original, the less of a parody it is (not weird al), the more you will struggle to use fair use as a defense. suno generating music hits all the parts of "this is hard to defend" while also being in the the most litigious possible industry for fair use