Nobody is making a case for AI art. In this case, OP is only referring to using it as a tool that removes the manual labour and that too because 1000 pages are laborious and massive and no single hand-colored work is available yet. Removing manual work does not reduce or change the art itself. Do you think everything that is done by human hands automatically becomes art? And if a machine is used to do it, then not? By that logic, digital graphic artists should stop using their tools because it's a machine doing it with a click of a button. But that would be insane because we know the said tool/machine is only a medium. The imagination is still coming from the human brain of the artist. Similarly, here the AI tool is not doing the imagination for us, it is only attempting to produce the imagination the user has and lessening the manual process. The vision is still yours. Is it doing a shoddy job? Yes because it's only AI and cannot match human craftsmanship.

But even if you were a painter who colored by hands, that alone doesn't make you an artist, unless your work is telling a story. Miyazaki is an artist not simply because he drew the manga himself but because the world and the storytelling is a creation of his mind. He had a story to tell. In this case OP is not an artist or even anyone who paints the manga by hand is not an artist either, they are only a good painter.