3.1.3(b) is probably what they're obliquely referencing, but I was once told by an Apple App Reviewer that "not all of our rules or guidelines are published" which caused me to have an aneurysm and shout "THEN WHAT'S THE POINT OF THE RULES?"

"You must not directly or indirectly target iOS users to use a purchasing method other than in-app purchase, and your general communications about other purchasing methods must not discourage use of in-app purchase."

2.3.10 has a bit about not including "irrelevant information about Apple or the development process." It's under the Metadata section, but still:

"2.3.10 Make sure your app is focused on the iOS, Mac, Apple TV or Apple Watch experience, and don’t include names, icons, or imagery of other mobile platforms in your app or metadata, unless there is specific, approved interactive functionality. Make sure your app metadata is focused on the app itself and its experience. Don’t include irrelevant information, including but not limited to information about Apple or the development process."

https://developer.apple.com/app-store/review/guidelines/