I've added a Steam key option to the game and it should be functioning as intended. I can see that several people have claimed Steam keys for the game already, so I pesume that it's all set up properly. If it's not working for you, it's probably an issue with Itchio itself.
If you want you can email me (ebihimes@gmail.com) with proof of your purchase on Itchio and I can send you a key there.