This would be super cool. I could see a lot of ways to build it, but of course the strength of extensions would be allowing the client to interact with content on the page. #[0] 's highlights would make sense for this, not sure what else.
Discussion
I think he wants to put an entire social client inside the client codebase and have screens for that, which looks great to me. Then people don't have to trust that a webpage is malicious anymore, and they can bypass CORS, get notifications and stuff like that. Like https://fraidyc.at/ maybe.