objectively its color contrast ratio on a white bg is more than high enough to meet web content accessibility guidelines

Well, I disagree.

Always depends on the color of the background : )

That depends on the medium used to view the web. On my KoboHD e-ink reader, grey text is not legible. It's sad because thousands of web sites that would be gorgeously legible if their text were #000 are not. But this is more of a browser issue; Kobo should disable colors IMO. And this is something that people can do on regular browsers.