You're definitely right about them prohibiting devs from pointing to a web browser to subscribe. They can't link, can't even use language that explains WHY you can't subscribe in the app (if the dev decides to forego using Apples payment system entirely). I think that THIS was what Apple recently was forced by a judge to relax, after the Apple v Epic case.

I must have been conflating the policy that they ABSOLUTELY did have (but only til 2011) and the outside links issue. Hughes Hubbard

Under the February rules, if developers wanted to use content purchased outside of the app, they also had to offer the content for in-app purchase and it had to be offered at the same price or less than it was offered elsewhere, despite the fact that Apple takes a 30% cut.

Macrumors

Apple has quietly changed its guidelines on the pricing of In-App Subscriptions on the App Store. There are no longer any requirements that a subscription be the "same price or less than it is offered outside the app".