It's a shame but he gave people notice (instead of just stopping development) and recommended people request refunds. This is nowhere close to being a scam.
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.
Without an updated blocklist, this app will quickly become useless.
"The app is completely my code, using a copy of Ghostery’s tracker database hosted on my server that the app periodically checks for updates."
So since he's abandoned the app, and broken his agreement with Ghostery, I'd assume he'll no longer be receiving an updated database from Ghostery.