I don’t understand what you’re talking about. An open source app with a subscription service does not make it a proprietary app.

There are plenty of open source services/apps that costs money. Open source ≠ Free (as in money)

In the link, the dev even said:

Subscription using Patreon will also be available, but you can only download it on GitHub. It will not contain any Google Play services.

They haven’t mentioned anything about the app being closed source. Please do not spread misinformation.