You're kidding right? That's such an endlessly common thing in anime art, the idea of that being your reasoning is just.... Also, the artist lists all their hardware and software, as well as condemning AI art.
AI can't yet generate legible, coherent text, let alone text relevant to the picture, such as a character's name. So at the very least the background is not AI-generated.
The tag was (probably) added because the wiki states that they post AI-Generated content. One of their other images on pixiv has the following caption: "Created by Kusama Hiroshi & NovelAI." Which is probably why the wiki entry says that. Definitely makes all of their subsequent artworks at least slightly suspect.
