It seems like what Medium is doing is creating a subscription-only publication of their own, competing with existing Medium publications. In the email, they made a big deal out of the idea that most users and writers would still not subscribe — which means that, if they want to keep their existing audience, existing publications will be unaffected.
I’d like to see them open up a submissions for fiction and poetry, if only because fiction and poetry publications on medium have short, rare, and unpredictable submission windows and typically pay peanuts compared to nonfiction publications. (For instance, How We Get to Next — not even a huge publication — will pay out nearly $300 for a relatively short article that meets their standards, while fiction of similar length and quality on Electric Literature is capped at $50 if I recall.) If their experiment with nonfiction is financially successful, they could probably afford to assign a couple people to a fiction slushpile and keep submissions open perpetually.