He was replying to a post that matches the price of a computer to a car, with the implication that that's expensive. He replied saying that a rMBP "starts at $1299", and he was disagreeing with the post he was replying. Thus his implication was that this is a cheap price.
I'm willing to retract that entire line of thinking, because the author never ever actually said anything about designing for Retina hardware, so why are we talking about it in the first place? Their explicit approach was to make Medium's typography work on as many platforms and devices as possible, and this OS X (not Retina, just OS X) bug was an edge case that they missed.