"Developers may apply for an entitlement to provide a link in their app to a website the developer owns or maintains responsibility for in order to purchase such items."
Thanks and wow, they’re really doing everything imaginable to make it difficult to include such a link. And their rules say the link cannot contain any additional parameters so it rules out the possibility of just linking directly to a payment form. You would have to log in first, then pay.
Devs just won't add links when informing their users that it's cheaper on their website and Apple can't do anything about it. They are not allowed to ban it according to this ruling.