To me it looks kind of similar to https://svbtle.com/, which is a network of chosen writers with multiple blogs aggregated to a single site.

Medium.com is a publishing platform for the sharing of ideas and experiences.

It's founded by Evan Williams[1] of Blogger, Twitter etc.

[1] https://twitter.com/ev

and Biz Stone, another twitter cofounder

Biz Stone is a founder of Twitter, Obvious and now Jelly. I don't consider Biz Stone to be a founder of Medium as it operates independently of Obvious.

Obvious have invested/incubated Medium, Branch, Jelly, Lift, Beyond Meat, Good Fit and Neighborland. (There may be more but I'm pretty sure that's all of them)

Medium, Branch and Jelly are what the Obvious founders are now focusing on (although they sit on some of those other startups boards):

Ev - Medium, Branch - Jason and Jelly - Biz

You may very well be right, I had assumed more involvement on his part based on this article: https://medium.com/obvious/e74637f2fe22