The demo looks good but look how much fiddling he had to do to get the images the way he wanted them at certain points. There are so many little quirks that you can't really solve intuitively, you end up having to brute force a solution every time.

He even dealt with a number of non-image quirks and he probably coded the editor. Like when the paragraph and header were both bolded and he had to go back and fix it.

Medium is also highly targeted to blog posts too. A lot of non-technical users somehow want the ability to edit the content on the pages of their web site that might have fairly complicated markup/css to get the desired result.