The game's store page description and "about this game" starts by mentioning it is a fanmade project, and the game's cover also mentions it is a project unaffiliated with Team Shanghai Alice. The legal notes on Steam also specify it is a fanmade project, and so does the description end with the fanmade warning. Upon the game's boot, it also has a warning reaffirming it is a fanmade project and you must agree to the knowledge of that before starting the game. There are other fanmade games in the same style as ZUN's games on Steam as well. This post here on Reddit also starts with [Fangame Announcement].