Several years ago, I led two different front-end web teams at a large, well-known company. I wrote a lot of JavaScript then. In fact, I wrote the core JS framework for one of the apps. I probably understood "this" at that time. I couldn't begin to tell you now, because I've been working on hit iOS games since - probably without knowing a lot of the nuances of Obj-C at times! There was no disaster back then - we shipped high-quality sites on time. There would be no disaster now, because if I went back to JS I'd crack a book.
The most important skills I bring to any project are a lot higher-level than the details of a particular language. I haven't made my career on one language or platform, but dozens of them. For the details, I work with reference material handy. After a month, I probably have a good grasp of whatever I'm working with. A year after that project, I probably don't, but I could bring it back easily.
The ECMAScript committe are taking steps to make "this" more useful, but the current semantic is so insane that I don't it's particularly surprising that people who aren't language lawyers don't understand it.
(*) Given that Javascript uses a single-threaded model.
It depends on what you mean by "down cold," obviously, but if your standard is "either they have the semantics of the 'this' keyword down cold or they're incompetent" then virtually all JavaScript programmers I know (including myself) are incompetent.
