I was talking to the developer, sorry for the confusion, haha. If they want their game to work with the Itch.io desktop app, they need to tell it which files correspond to each operating system, and right now, both files are marked as not being compatible with any of them.
As for you, just install the game manually by going to Itch.io in your browser, clicking the arrow in the upper right, going to "my library," and downloading it there. Updates are infrequent enough that the desktop app doesn't really offer much benefit over a manual installation.