I'm not advocating for Apple here, this particular post just strikes me as being an odd exception.
Just a wild guess here, but it's likely they have some language like "Click here to subscribe" which sends users to an outside link. That is expressly forbidden.
UPDATE: The MacRumors story is more coherent and perhaps has a clue to why the app got rejected: https://www.macrumors.com/2020/06/16/apple-threatens-to-remo...
Maybe their odd "Help Me" page is shooting them in the foot. by comparison, Fastmail has a simple "Can't Log In" button which takes you to the Fastmail support page on lost passwords.