Hi, this is an issue with itch labeling this as a "Steam key" but it's got nothing to do with Steam at all. I'm sorry about that. Since you purchased, you should be able to download the file directly from the page here on itch. They key you received will be entered into the plugin itself when opening it for the first time.
Sorry for the confusion! - itch has no method to get rid of the word "Steam" so it's kind of stuck that way.