Simply put, things that are better for someone who needs accessibility are also better for everyone else.
Maybe I want to watch something while muted. Maybe I just don't WANT your site screwing with my font size. Maybe it is a lot easier to tab between things than using the mouse. The list goes on.
But in the real world, this isn't always the case. The article itself is a great example. It uses giant fonts and lots of whitespace, which is good for people with poor eyesight, but it leads to a choppy, hard to follow article. It also forces you to use navigation to follow a chain of thought.
I said "needs accessibility", not "needs large font". Accessibility means that if someone needs to set the font to size 100, they can and the system won't break because of it (e.g. text won't be cut off with no way to see what was lost). It doesn't mean that the default choice is some kind of lowest common denominator that is weird for everybody.
The only exception where defaults must be pessimistic are the settings menus themselves. For instance, a button that increases font size must be large enough for anyone to see; a button that changes language cannot be only labelled with the text of a foreign language.
Maybe I want to watch something while muted. Maybe I just don't WANT your site screwing with my font size. Maybe it is a lot easier to tab between things than using the mouse. The list goes on.