I'm not sure where the ai cg tag comes from, on the steam page the only ai content mentioned is music. But granted, the only thing I can find is a staff member having "character design" credited to them, and another "background design", also someone being told at a game convention by a team member that they "enjoyed designed designing and creating Lottie's character" but that's anecdotal rather than proof (not that I care about the guy, don't even know him, or that making ai gen music is any better than ai gen art).
That aside, the challenge stages don't have anything attached to them so no real reason to do them aside from playing.
Also they put a message on their twitter about a content update on august 18th.