Doesn't anyone realize they just wrote that article and disabled ie9 to attract hipster freelance designers and developers? The whole thing is basically a PR stunt since they realized 99% of their hits were from Mac anyway. Especially having their article on Hacker News was just to get attention from the large freelancer following on HN.
I'm no fan of IE. However explicitly blocking IE9 is just idiotic for any other reason. They could, at worst, just have a subtle notice letting people know it is optimized for Chrome/FF and their mileage may otherwise vary.
However, for the reason of getting Mac using hipsters excited, it was pretty brilliant.
(btw, I'm not a Mac or Windows user)
Disclaimers: (1) This is just a theory. (2) I actually like hipsters. I just recognize they can sometimes be manipulated. Like everyone, they have their buttons.