Yes, the first thing I've thought looking at the page on the computer was "I wonder how that looks on my iPhone." I've tried: not easily readable, for me.
Again, this statement just seems like cargo culting when taken as a rule.
The reason san-serifs were better on screen is that screens used to have lower DPI's, and thus can't actually show the serifs as well as on paper.
In the old days of CRT's and 72 dpi screens, the serifs would be distorted because the display can't show details fine enough. In those cases, san-serifs get you around the problem of low dpi screens.
Nowadays, with retina and high dpi displays, serif fonts can look as good as in print.
Just be aware of why these guidelines exist. It's not always a firm "it has to be this way" hard rules.
I actually like serif fonts on retina screens. The first "reader view" on the iPhone was serif and it was easy to read.
On the linked page the resulting size of the letters on the page is simply too small to allow easy reading. When the area to display text is so small, the designer shouldn't insist on the white margins or the comments on the side. On web page, the comments could be much better done in the "Wikipedia" style (showing on click). The design of the Wikipedia articles on the moment is actually good on iPhone. Much better than this, very natural to read.
