You didn't look hard enough. Also, the answer is right in the project's description.
See below:
Hey, thanks for your support :3. These releases are mirrors of our monthly Patreon/SubscribeStar releases, so they only receive critical updates. New monthly releases need to be purchased, just as our backers on Patreon/SubscribeStar would pledge for a new update. We largely provide our content on itch so people don't have to sub (and for people who can't use those sites).
Once the game is complete (and probably during the early access stage) it will be available for a single price.