Yes, that is how it works, we do one build per month on Patreon, so we do one build per month on Itchio too. So when you buy it you only get it once. We will do a Steam sale later on and that one will be updated regularly. We wrote about this on the post, you must have missed it:
Please be aware of the following!
- This purchase will not be updated monthly
- This is not early access for the final retail product
- New monthly releases will appear on our Itchio profile as they become available