That is poor advice which doesn't account for the difference between computer screens and ink on paper. The deepest black you can get on a monitor will not be as dark as certain inks on a page.

I'd argue that it does account for that difference, just not in an immediately obvious way. Screen displays are an additive color model and print is a subtractive one; screens shine light at you, paper has light shined on it. This makes for a very different perceptual feel; (completely) black ink on white paper is way less "contrasty" than #000 text on an #FFF background. WCAG accessibility guidelines suggest a minimum contrast ratio of 7:1 luminosity -- #000 on #FFF is a 21:1 contrast ratio!

In practice, I think we'd be better off darkening the background a little and lightening the foreground just a touch; #333 text on #F8F8F8 background still "feels" like black and white, but (assuming you've chosen a reasonable text size) it'll be a lot easier to read if you're dealing with long-form text.

I find it difficult to believe that the relative difference between #F8F8F8 and #FFFFFF matters at all relative to the the widely varying screen brightness settings on everyone's very different computers, tablets and phones in very different lighting conditions.

It's (generally) bad 'design' to fill areas with black, sure- but #000 outlines and text is just getting proper use of the limited dynamic range we're provided with.

#000 needs to be used tastefully, otherwise it can be visually overbearing.

Do you find this page visually overbearing because all of the non-downvoted comments are #000? I have never seen a page of text on a computer screen look overbearing because its color was #000 and not lighter.

What stands out more? The comments, or the names of those who comment?

Experiment, write this css into your dev tools:

    body * {
      color: #000 !important;
    }

No, but I probably would if the background were white.

Why?

#000 is easier to read on devices with that have dim backlights.