You're reaching here with the Wordpress example (.org isn't even applicable to this discussion -- it's an open-source CMS). Wordpress.com is a hosting platform. Just because as a subset of the options you can make a blog (vs. an ecommerce site, a portfolio, a landing page, etc.) doesn't make it Medium's competitor.
Visit their homepage. Do you see a content aggregator or something closer to Squarespace?
Wordpress is 100% a competitor to Medium. Probably their biggest competitor.
Even if you want to restrict to .com, WordPress.com (VIP) competes for many of the same users which Medium has been competing for.
I'm not saying what they've done is easy by any means, I'm saying that what they've done is not innovative and should be considered so just because they have a lot of users. Clearly a lot of users does not solely make a business successful, as evidenced by their layoff of 1/3 of the company today.
Also, innovation is doing something in a new way, whether by customers or product. And nothing they are doing is by any definition new.