"Separate all links with more than whitespace" makes it easier for everyone to find and click the correct link, rather than hesitantly mousing over a link and hoping it's right, or having to slow down and carefully click because the target is small and close to something else.

"Use a sensible default font size" - yes, people can zoom the page. But starting with a sensible large font size means they don't have to. That means your careful layout doesn't get moved around and possibly broken.

"Use a sensible set of high contrast colours; consider an easy way to switch to light on dark" - offing low contrast colours for text and background means that people are going to either not read, or are going to use scripts to replace all your colours. Some people prefer (or need) light on dark, and will use scripts to replace your colours. Your nice design will end up very different if I get to pick my own colours.

These doesn't sound like much, but there's a bunch of little optimisations that help accessibility, but which also help everyone. Tweaking a webpage before I start reading it may only take a few seconds. But first impressions count, and if my first impression is irritation at the bloody silly choice of light grey on grey colours and a bit of fiddling to make it readable I don't have a good first impression.

Also, compare the time it takes to make it readable with the time it takes to make a page load, and the efforts that people go to when they improve page load times. Half a second makes a big difference.

This is the ideal behind universal accessibility design. For instance, when you're in a loud and crowded place a person with average hearing abilities might have the same needs as a deaf person when watching a video. That's why museums regularly use open captions on video screens since they benefit everyone, not just the deaf.

As a side note, I bet this guy gets fantastic battery life out of his mobile device, since he can set the brightness to the lowest setting possible all the time. I guess I'm assuming that all that Siri and speaker amp usage doesn't drain the battery more.