And neither does Apple. You are free to offer both in app and out of app subscriptions and charge different prices. You don’t have to sell your app in the App Store, you have to distribute it.
This makes me wonder if they could charge $99/year on their website but $999/year via in-app purchase. Basically a price so high that no one would realistically buy it from within the app.
It technically fulfills the requirements specified in the review guidelines but I suspect it would be frowned upon.