Modern programming languages and frameworks are explicitly designed so that you don't need to see how your code fits into the whole. That's the whole point of building abstractions, because it's too hard to keep all of the details in your head at one time. Writing code is generally concerned with the low-level details, which are hidden from other parts of the system (and that's a good thing). When you ask someone a coding question, that's what you are testing. To the point of the article, yeah maybe there is some correlation between someone who is good at writing code and good at writing API documentation.
The complaint in the article is that the high-level conceptual documentation is lacking. The systems design stuff. The architecture. And while there are certainly people who are good at writing low-level code as well as understanding high-level architecture, that's not always true, as they are very different skillets (and I don't mean this as a slight against anyone, it's probable that most people who are good at one could become good at the other, but in practice, many people's jobs lead them to spend more time on one instead of the other, so that's where they build up skills and experience).
Anyway, the higher-level documentation is a very different style of writing. It needs to read like a book (as opposed to API documentation which is more like a dictionary or encyclopedia). You need to be able to pull information together from lots of places, and it must be presented in a methodical way, where (for example) you can't assume that the reader has knowledge of something before you have presented it.