This is not Steam. Nor a retail shop. You cannot give back things. There is no try before buy or any buyers remorse money-back guarantee. The time frame has little to do with this.
For that matter, can you get a refund for a patreon subscription? I tried to find out their policy, but I found nothing at all about the circumstances when patreon would grant such a thing.
I agree that they could be faster in telling you, that you have no eglible reason to request a refund.
So my advice would be, ask for clarification, if your one time purchase will give you any of the future patreon content or not. Subscribing for 15 months will cost you more and people tend to forget their subscriptions ;-)