Yes, the jam version has forced fullscreen enabled. I'm adding an options menu with a fullscreen toggle and the ability to change resolutions in an upcoming patch next week. If you haven't already tried, the app Reso usually works for overriding fullscreen mode for most apps.
Viewing post in Late Homework | 遅れた宿題 [Demo] comments
The app I have been using for years is Borderless Gaming which usually works fine but yeah for this one it just flat out didn't work. I will check out reso and see if it works! Edit; Can't seem to find reso anywhere. Edit2 It does do it fine but it Fullscreen it and changes the windows resolution. I'll just be patient and wait until next week Lol.