That's a very of odd definition of fluency. I could be unfamiliar with the concept of intransitive verbs or genitive case and still be considered fluent in my native language. Not knowing whether to use 'there', 'their' or 'they're' in a given sentence has little to do with your knowledge of grammar and basic linguistics. If you have to analyze a sentence grammatically to correctly use 'their' in "She went to their house", you almost certainly are not a native speaker of English.
I suspect that many people develop a greater understanding of their own language, and languages in general, by studying a second language. I still remember the "Ah ha!" moment I had in junior high school when it dawned upon me the doing word for word translation of English to French was almost never going to produce the correct result. My world grew much larger that day.
But my fluency in English was unchanged.