Honestly I think that if interoperability between different types of federated platforms Just Worked people would "understand" it more intuitively, because they'd see it in action instead of it having to be explained to them as an abstract concept.
I'm a fairly tech-savvy user, and I perfectly understand what federation is and how it works on a technical level, but even I find the UI aspects often confusing or downright impossible to figure out.
For example, I saw this thread from my Mastodon client because I followed the ActivityPub topic on this forum from my Mastodon account. In my Mastodon client the posts appear as boosts, but mostly with broken HTML formatting. I clicked through to this forum to read the thread properly. I then wanted to "like" / +1 a post, so I copied the permalink into my Mastodon client like I would for a post on a different Mastodon instance, but it couldn't find anything. In the end I had to log into my local account on this forum instead.
Don't get me wrong, the UI problems are very difficult to get right. But I think they're more important than documentation. When it Just Works, people will grok it and documentation/explanation won't be needed.