My interpretation is that itch.io has tons of products that are just PDFs, WAV files, 3d models, whatever. When a download file is not marked with a target platform, it might be very hard to impossible if the thing even can run.
You might want to try and point the missing metadata out to the game devs themselves. Having correct metadata is in their best interest, after all.
In the meantime, you can just download the files through the web interface instead of the desktop app.