Apple does allow links to external payment options in some cases (see App Store Review Guideline 3.1.1), and sometimes rejects apps for links that it itself say

Apple does allow links to external payment options in some cases (see App Store Review Guideline 3.1.1), and sometimes rejects apps for links that it itself says should be legal, and is even legally required to allow in some jurisdictions. Which is not surprising, app reviewers spend only a few minutes looking at each app, and don't always understand the current rules.