This is absolutely correct for books, but a blogpost is not a book. The most significant reason to use sans-serif for UI text is that screens with less DPI tend

This is absolutely correct for books, but a blogpost is not a book. The most significant reason to use sans-serif for UI text is that screens with less DPI tend to utterly destroy serif fonts with small font sizes. Serif fonts just rely that much more on high DPI values like on a book. If you somehow have an audience that's exclusively on Retina displays (e.g. for iPad/iPhone apps), I would be less hesitant to try out a few serif options. But designers tend to forget that e.g. not all desktop users are on color-graded 5K iMacs.