I'd be a little surprised if Apple will allow them to do this. It's kind of like credit card companies requiring stores to charge the same price regardless of if you use the credit card (which has fees) instead of cash (which doesn't). Charging different prices is a negative signal for Apple's ecosystem ("Why can I get this cheaper if I have an Android phone? Maybe I want an Android phone instead next time....")
While I might argue that the negative signal shoooould exist, I'm surprised Apple doesn't have it in its terms that you can't do that if you want your app on the Apple App Store.
Also, could we please amend the title? It's not a flat $12.99 a month for everyone, and it doesn't even match the headline on the actual article.