I did just test turning off webfonts on Medium and I still honestly can't see any huge problem with my default system typeface. This could possibly be because I have nice defaults, but at the same time, I think the good readability of Medium is far more about the overall style/layout of the page (particularly the decent large font-size), than about any particular typeface.
Example 1 - normal Medium: http://i.imgur.com/D4hYBPy.png
Example 2 - default system fonts: http://i.imgur.com/Jvnuhkm.png
Whilst I can see the difference in typeface, I can't honestly say that I'm experiencing any huge readability changes. Perhaps if I were to switch to a Sans of some sort, but that's not exactly a huge decision that needs to take a lot of time and fuss. By far and away the most important thing that makes this page readable and nice is the font size, closely followed by the line-width.