Would it be considered an ad if instead it just said 'hey new thing now you can create and print photo books right in the app!' without mentioning price?
Yes, because it still wants your money. Saying "Now app X has a feature to do Y" is fine, but when it requires additional purchases to "do Y", it is an ad. If Hacker News would display "did you know you can subscribe to read grahamburgers next reply", but I have to pay before I can read, it is an ad.
The philosophical question that arises from here, now, is that if someone really cared about that offer to be introduced through the app (like a new feature notification), would it still be called an 'advertisement'? But then, to know whether you "care about" this feature to be introduced in the app so that it wouldn't be an advertisement, they'd need to track what you care about and what you don't. But people have issues with tracking :). Not supporting or opposing anything here, just thinking aloud.