Elizabeth,
You have brought up an interesting question: Exactly what does "fluent" mean? For example, I have a Japanese guy I work with, and his English is excellent, but he is not fluent.
When I think of "fluent," I always think of my mother (of all people....) My mother was fluent in English, yet she was not an educated woman. (She never graduated from elementary school, typical for early last century.) She could not have any type of intellectual discussion on any intellectual topic, and she never used or understood high'falutin' vocabulary like "inter-stellar space" or tri-glyceride fatty acids" (words an educated person, but not an uneducated person, would be expected to know. Anybody remember Archie Bunker and the way he frequently slaughtered big words...?) My point is, as you said, a person can be fluent without knowing any of these 64-dollar words. This is also true of some grammar forms.
This, then raises the question of what makes my Japanese co-worker non-fluent? Even though his English is excellent, he does not know vocabulary and grammar that even Archie Bunker would have been expected to know. (The phrase "would have been expected" -- what I call past-future continuous perfect -- is a good example of the grammar that I am talking about.)