Semi-offtopic, but is "medium" twitter's version of "posterous"?

I just noticed that to leave a comment at a medium site, a person needs to login through twitter. And the only social media support they seem to offer in my 15 second random survey of their front page is via twitter.

Ah, a bit of googling shows medium and twitter share co-founder Ev Williams. http://mashable.com/2013/04/17/medium-acquires-matter/

So is there any formal ownership between twitter and medium?