I can borrow a controller to set up, and that probably is the better way to play this anyway, but I opened it with keyboard (I'm primarily a keyboard gamer, I played Celeste with keyboard) and I noticed that WASD seem to be mapped to something, because they move the logo around. Also X and numpad . close the game, but I can't find any buttons mapped to A..
So it appears impossible to start the game, unless there's some key I missed trying. Anyway I'll probably pull out a controller anyway, but I didn't see anyone else mentioning this yet.