I think that the authors ideas about what it means to know a language are implicit to the whole article but never directly discussed. There is knowing the meaning of a word and then there is knowing the contextual usage and conotative meanings. On their face, "expensive" and "costly" have the same meaning, but in many circumstances they aren't fully interchangeable.
To me, your "mother language" is simply the language that your mother, or other primary caregiver, spoke to you as a child. It is certainly plausible to have more than one in multilingual households.
A native speaker may not have spoken that to language with their parents, but they learned it natively from other speakers at a young enough age that their phonemes adjust.
In my opinion, native speakers tend to have a much deeper understanding of the meanings and connotations of the words/phrases they use but may indeed have smaller volcabularies than fluent, but non-native, speakers. The difference between "costly" and "expensive" is obvious to a native english speaker, but might not have been learned by a fluent speaker with a larger volcabulary.
If you listen to linguists, grammar is descriptive not prescriptive. Grammar is not a fundemental trait of language, but a model of language use that helps us think about how language is used.
I do think it is important to be aware of how we use language to enforce economic and cultural segregation. Language use is often used as a proxy for class and education and those who don't fit the "standard" are faced with discrimination.