Some of this article is poorly researched. Rocket.chat is not moving over to the protocol, it's just implementing federation in such a way that Rocket.Chat servers can communicate with Matrix servers. It's still its own (heavily flawed) thing.
I think the distinction the grandparent post is making is that RC doesn’t speak the Client-Server API, so you can’t point a random Matrix client at it. Honestly, I think it matters more that you can talk to it transparently via the Matrix network than connect your favourite Matrix client to it.