I agree that this part of the article is not well developed, and I should have probably rewritten it. What I was trying to get at is that Flipboard has a hierarchy that is unique: The Actor owns a Group, with the Actor being the highest level, and the Magazine below it.
While you could probably map a Flipboard Magazine onto a ActivityPub Group similarly like a Kbin Magazine is a Group, the intended usecase is still quite different, even though they are both link-aggregators. I dont think this is an insurmountable problem per se, but I dont agree that Kbins Magazines are super similar to Flipboards Magazines. If you wanted an fediverse comparison, I think Flipboards Magazines are actually way closer to the Postmarks.