Interesting. I have on gripe with that page is that the examples of bad contrast say "NO CONTRAST" while the good examples say "HIGH CONTRAST". The bad examples should be more accurately labeled "LOW CONTRAST".
Yours is a definition of contrast, but not the one used in design or vision science [1, 2], where contrast is defined as a difference in luminance or color between regions of an image that makes objects and textures visible. If an image has no contrast, it is a solid patch. If an image has low contrast, the brightest or most colorful regions are not too different from the other regions, as if seen through a dense fog. If an image has high contrast, the brightest or most colorful regions are quite different from the other regions.
Here we're talking specifically about "visual contrast", which has a meaning in perception slightly different than the common usage of "contrast".
Visual contrast is measured on a spectrum that starts at zero and goes up from there - a box of a particular color inside a box of the same color would have no contrast, but if there is any difference in the color values then there is at least some visual contrast.
Wikipedia isn't a great source for definitions, but has a reasonably thorough take on this https://en.wikipedia.org/wiki/Contrast_(vision), going into the details of contrast sensitivity, which deals with almost-imperceptible levels of contrast.
I just wish they'd fix their site so it doesn't break the layout horribly if someone has set their default browser text size to something other than 16px! The irony of a site that is about maintaining accessibility for those with less than perfect vision breaking under one of the most likely changes to be made by someone with less than perfect vision is rather awkward.
So that more people can spread word about how low contrast/less readable text hinder accessiblity.