I do a lot of web development, and a lot of that is stuff designed for desktops. I do my best to make sure it works as well as can be expected on mobile devices as well, but I completely agree that it does make things more complicated, and that often you do need a different design, or at least a very flexible design.

But this is not a complicated web application, this is a web document. If they had just added the right meta tags, it would have probably been enough. I'd be surprised if you really even need media queries for a page like this, just judicious use of `rem` and `%`. It's also very easy to test - just make your browser window thinner. Or open devtools on the side, and drag it around a bit. Or there's even a button in most browsers' devtools where it'll show the page in different common mobile and desktop sizes. I think lighthouse usually defaults to mobile too. (Not that lighthouse is necessarily all that valuable of a metric, but it's there, it's a couple of clicks away.)

The point the website is making is that accessibility is important. Light text on a light background might be aesthetically pleasing, but it's difficult to read for many people, and as developers and designers, we have a duty to make our work accessible. I fully agree. But mobile users - and especially mobile-only users - are also an important demographic, and deserve accessibility considerations as well.

Especially when those accessibility considerations would take at most half an hour of work.