Earlier quoted context omitted.
Yeah this case seems incredibly obvious, and I feel like any suggestion from the developer that they aren't violating the policies is disingenuous. Whether the policy is a good one or not is a fair question, but not one that App Store review will (or even can) answer, and trying to do anything there is barking up the wrong tree. The right place to challenge this sort of thing is likely with organisations like the FTC…
The developer had a meeting with Apple representatives where they assured him that tipping profiles would be okay so he adjusted his app so people would tip profiles instead of posts. Then they still rejected the app.
I can't find the bit from the dev you're referring to so don't know if they have screenshots of it in writing or something, however, what I suspect is more likely (based on my experience talking to Apple reps about similar things) is that they gave a very charitable description of their feature, and the rep gave a non-committal "sounds like it could pass, try submitting it".
I don't get why the developer is so convinced that their tip system for tipping people based on their activity on their app is not tipping for "digital content". Unless you can only tip randomly with no control over who it goes to, or tipping is for the user's activity off-app, digital content (i.e. posts etc) are literally the only thing you could be tipping for.