I have been using the itch.io app on desktop (in Windows 10). I prefer the desktop app over the browser site since installing games from the app keeps it all nice and tidy in a hidden folder instead of cluttering the downloads/desktop.
I wonder if it's an issue with certain file types--shortly after I made the original comment ,I was looking at a friend's game that also showed this message in the app, even though it was definitely a Windows-compatible executable. Now that I am looking at your game on the web browser, it lets me download it just fine.