(That page ends up referencing, among other things, http://en.wikipedia.org/wiki/Satori , which may go well with sushi.)
'Grok' has always seemed to mean to me something kind of like this, where no further epiphanies are possible.
I've never heard of anyone using grok to mean something along the lines of "mastery".
I'd say 'grok' generally means that you truly understand something in a fundamental way, rather than just being 'competent' or 'masterful' at that thing. It's a crucial step on the path to mastery, but mastery might still require a lot of practice.
I find 'Your problem with Vim is that you don't grok vi [1]' to be a classic example of proper usage. It explains fundamental principles of vim's 'grammar'. Once you've read and understood it, you 'grok' vim. But even though this knowledge is crucial in being a 'master', it does not make you one. That would require a lot of practice (in this particular case, for me, a few weeks).
I've had similar experiences with functional programming. There was a point, after much reading, where I felt that I truly 'grokked' the approach, but I'm still far away from being a master at functional programming. Old imperative habits die hard.
[1]:http://stackoverflow.com/questions/1218390/what-is-your-most...