Looks like it comes with a steam key if you buy it here on Itch.io as well. Which I just did, because I like that I can download and _own_ my games on itch and the dev gets more money, but I also like steam's social features/achievements/workshop.
Best of both worlds, and than you for that Cairn4.
Edit: Download file was labeled moonbase-win.zip instead of mewnbase-win64.zip. Checked the other two out and osx is also moonbase-osx.zip, but linux one is mewnbase-linux. I have no idea if that matters but just a heads up in case it means you didn't feed itch the file you intended.