The author claims that a strong knowledge of development tools is a good indicator of general proficiency, but what if it's that learning how to use your editor makes you proficient? After all, nobody gets familiar with Emacs without inadvertently becoming a practitioner - if not an enthusiast - of Lisp!
That happened with me, and I'm now a big fan of functional programming, Clojure, Scheme etc., and that experience and exposure to a particular way of writing code has undoubtedly made me a better programmer. It sounds like you've had a similar experience with Vimscript :)