One thing about Emacs (mentioned in another comment in this thread) is that it is more than an editor but mostly used as an editor. It does not take 10 years to learn to use it as a better notepad (I mean, better pico... whatever), but mastering its existing (elisp) code base and adding to it (more than the trivial customization) takes a while indeed.
The article mentions some of the general reasons why it takes a while. Another one is that few people actually set out to "learn Emacs" as such, they just do it as an afterthought while working on other projects, and therefore not in a consistent and steady manner. So there may be months between "lessons" and, therefore, 10 years to complete a course, so to speak.
The part of Emacs that takes years to learn is integrating it with everything else. The core editor doesn't take long.