If you go to itch's website instead of the app and you click to download, it should bring you to the download page and you should be able to redeem it there. That's how I got mine, just waiting to redeem it now 😊😊😊
We ran out of keys but we've just added more! The game isn't live on Steam just yet as it's pending review but as soon as it is you should be able to redeem your key :)