An app cannot send you push notifications unless you have opened the app and allowed push notifications (or gone in to settings an explicitly enabled them, which seems unlikely). So it is incorrect to say "the app wasn't used". It was obviously used, just maybe not in a long time.

Sorry, I'm not trying to nitpick. But it seems like all you've said so far is "An app which I downloaded and enabled push notifications for sent me a push notification recommending a video I didn't agree with politically". The term "extremist content" can mean basically anything depending on who is speaking.