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,

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.