Clickbait phrasing.

tl;dr:

1) Services can track you based on the data you've given to the app.

2) Obviously, this doesn't stop when you uninstall an app.

3) It's possible (using push notifications) to determine if users have uninstalled.

4) The service behind the app can then track you with a specific "uninstalled the app" status.

As an aside, I dislike the fact that "app" has become such a catch-all. It introduces confusing language, like in this example. The "app" didn't track you because the "app" was a client-side binary for interacting with a service. The service tracked you from the beginning.