Despite claiming to promote high contrast text, this website is medium contrast at best. Their "black" text is #191919, not #000000 as it should be. Printer toner and carbon black based inks are frequently darker than #000000 on a typical screen, and nobody complains about them being too dark.

In the case of white being darker than #ffffff, you can at least argue that most users have their monitors configured too bright (it's too bright by default because that looks better for a short term comparison, and most people don't change the defaults), so darker whites are compensating for that. There's no good reason to make black text gray.