Their "contrast" section belies their determination, since the text there is grey on a white background, and true black on white would unquestionably be more readable. (looking at UI designers more generally:) I'm so sick of UIs that have repetitive, huge, contrasty, readable headings... only to have the core useful text be grey-on-something, tiny, possibly italic, and so on. Idiots, and they are legion. More subtle issues like older folks having lower visual resolution, and many folks (men mostly) having significant color blindness, never seem to occur to UI designers as a problem.
Black on white is a hand-me-down from print that made it's way to web without anyone giving it much thought. It is indeed too great a contrast for back-lit screens. Google results also show the page descriptions using grey on white. Surprising that so few people know of this.
