There's several ways we can tell. First off, and most importantly, there's nothing on ZUN's website about it. Second, did you see the background to Stage 1? It was flat, ugly, and looked like something drawn on construction paper. Touhou Project games always have beautifully-rendered backgrounds with 3D effects and such. Third, while we'd all really like to see Mima return, it seems out-of-place for her to suddenly reappear as a playable character. It seems far more ZUN's style to put her in as a stage 3 or stage 4 boss to re-introduce her. Fourth, I doubt he'd make the stage 1 bullet patterns that intense. Fifth, if you look carefully in the beginning of one of the videos, it says 13th Imitation Shrine Maiden. Anyone who noticed this in the beginning of the video should know right off the bat that this isn't made by ZUN.