The music is 100% ai generated. You can hear how some of the generated instruments try to blend in in a way that isnt good. Plus there's blatant rip offs of other songs in some of the tracks. And no, it's not synthesized instruments, it's really simple to tell how it'd be synthesized vs otherwise.
I do like the art style, that part (aside from the backgrounds) isn't ai generated, it's just the music.