Doesn't it say when you go to flag an image that good but "off-topic" art isn't valid to flag?
No, that’s only mentioned under cosplay photos. Anime cosplay isn’t considered off-topic on an anime image board. Pretty much none of the current approvers would approve it, though, even if it’s not against the rules.
Flagging this image for being off-topic is okay because it’s not remotely anime-styled and not related to any anime-styled copyright either.