The content at that link does not justify the comment;
@feld
> Matrix devs admitted to leaving a side channel open
... as a quick skim of the intro and addendum make clear. This is the equivalent of the attention-seekers who claim XMPP is broken and unusable because metadata. Clownshoes indeed.
But if you're so motivated to slam Matrix that you'll make a claim that strong, on the basis of evidence this flimsy, I doubt anything I say will comvince you.