While that's true, keep in mind that not everyone who bought Peace know who Marco Arment is. I don't even think he released the app using his name.
Customers who bought the app only because it was the top app in the App Store at the time have no way of knowing the developer stopped supporting the app. They're screwed in the long term because they'll think they're using an ad blocker when they don't.
I think the smart way to go about this would have been to make the app free and let users know via the interface that the app will no longer be updated and users should ask for refunds. I realize this could make the app even more popular since you make it free, but I think this is still a better way to let users know about the future development of the app.