Plait explains the math behind pixels and resolution. Specifically (and I'm paraphrasing liberally here), two nearby objects observed from a far-enough distance appear as a single object. Plait uses the example of an oncoming vehicle. Is that one headlight or two? Is it a car or a motorcycle? As it get closer, the lights separate and you recognize it as a car. This is because, Plait explains, over a longer line of sight the two objects closer together have a smaller angle separating them, making them harder to distinguish from each other.
So what does this have to do with the iPhone? Well, Jobs claimed that our eyes can't differentiate between neighboring pixels presented on the Retina Display when viewed from a distance of 12 inches. Soneria claims that isn't true, and Plait did the math to find out. He concluded that Soneria's numbers assume a user with perfect eyesight. Since the vast majority of people lack perfect eyesight, Steve's claim is true: Their eyes will fail to resolve those tiny pixels at a distance of 12 inches.
Technically, both men are correct. Steve may have exaggerated since those with perfect vision will see pixelation at 12 inches. At the same time, Soneira is nit-picking, as the vast majority of users do not have perfect vision.
I hope this, as Plait says, "makes things clear."