No you shouldn't have to buy the game again. If it's showing you that, then it's an error, probably because for awhile the game had a different purchase method. I was unaware it was asking people to pay again until you said something. It means I will need to update it directly through a different method. I did not upload the "actual premium version here", as much as attach it from a previous post because Itch allows me to do that. I didn't want to spend extra time uploading it directly because I had hoped it would be fixed by the end of that day, and I wanted to upload the new one, but the programmer didn't get back to me yet.
Tl;DR there should be no issue with the Itch launcher, it just wasn't updated with the old version through that method yet because at the time I was hoping to update it with the new version, and thought the old version it allowed me to attach would be free for those who bought it as temporary stopgap.