Linguists and lay people answer this question differently, but that’s because they have different focuses on what language competence means.
A linguist thinks of language as a rule system—a grammar, and a lexicon. As far as a linguist is concerned, the grammar is the common property of the entire language community: if you are a native speaker of the language, then by definition you know all the rules of the language.
A linguist knows that the vocabulary of a language is open-ended, and no one can know all of it; but they also know that vocabulary varies by register, and that to be competent in the lexicon of the language is to use the appropriate words for the appropriate register. If people don’t look at you funny when you speak in a given social context, then you have all the vocabulary you need.
So to a linguist, because of the way they think about language, all native speakers command the language equally well.
The reason a lay person does not hold that view is that lay people introduce value judgements in how they think about linguistic registers (because they use language in a social context), and they prioritise some registers over others as worth commanding. In particular, lay people are concerned about how well people command the prestigious variants and dialects of a language.
Those variants and dialects are not native to all speakers of the language at large, and many speakers have to learn them explicitly, as “non-native” users. That’s why it is meaningful to say that some people know them better than others.
Lay people also appreciate good command of style. They appreciate adept rhetoric and subtle usage of words. These are skills which some people will exhibit more talent or training in than others.
Linguistics does have the tools with which to understand the display of that skill. But linguistics has been reluctant to prioritise this as an aspect of linguistic competence. Mostly, because the evaluation of these rhetorical skills is culture-bound, and subjective, and communicative competence is less so. (Although not as much less so as they prefer to think: culture certainly plays a role in communicative competence too.)
The evaluation of how skillful your rhetoric is is the kind of evaluation that linguists are more comfortable leaving to literature studies.