Prince's lawyers were using whatever angle they could think of. Clearly an attempt to distort the meaning of the compilation concept. A photograph is not a book of trig tables. It makes as much sense as plagiarizing a biography or history, adding to it slightly, and then deeming the original book a mere compilation of facts.
It is of course revolting that Prince would be willing to crassly remove all artistic value from another's work, but then he was willing to rip off the work in the first place, so I guess it's no surprise.
How disastrous it would be if that compilation assertion prevailed. Would an unmanipulated landscape shot be a mere compilation? A straight, natural light portrait? A war photographer's work?