More explicitly: most professional print documents are done at 300 dpi. Since retina is 326 ppi, it's safe to assume that if it looks alright on paper, it will look fine on retina.
It's worth noting that it doesn't really matter though. Most iPhones are pre-retina, and this will be the case for at least several years. So I can't think of a medium where it's okay to assume this level of screen resolution. The author's point still holds.
300 DPI is the resolution recommended for images that will be printed on offset printers (like, for a magazine). The text - if black - in these applications will be turned into crisp vectors, so the DPI is in effect limitless.
I believe laser printers do print at something like 1200 DPI, like you're saying.
It's worth noting that it doesn't really matter though. Most iPhones are pre-retina, and this will be the case for at least several years. So I can't think of a medium where it's okay to assume this level of screen resolution. The author's point still holds.