I don’t really know … from my understanding the data is still leaked among the server instance users. So if the instance is not federated, maybe it’s only leaked within the instance itself, what is still a problem of design.
It would be good to have an update of this analyze as they did some improvement in the meantime. But so far I see Matrix as a very obscure alternative to XMPP, without significant advantages excepted a uniformized UI. So I would disrecommend to use it.