Personally, I think it’s fine to say that X speaks Matrix if X speaks the Server-Server API, like Rocket.Chat or Gitter does.
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.
