I've had this issue before. I run a feed reader called Pine.blog and App Review has rejected me in the past for content in the feeds the test account was set up to follow. These feeds don't originate on my site or service.

I had to schedule a call with app review and almost needed to escalate the call. It seems like App Reviewers aren't familiar with these kinds of apps anymore. They're used to silos where the app developer could simply ban or reject the content. You need to explain to a human that you're not providing a silo. This is a reader app for another community's site.