It actually means someone who grew up speaking language X with their parents, but they grew up in a country where another language is spoken. This person speaks language X fluently, but they speak the language of the country they live in better. For instance: José's parents are from Chile, but José grew up in Sweden. He speaks Spanish at home and he's fluent in Spanish, but Swedish is also his native language, and it is the language he speaks in school or at work and so on. He is a native speaker of Spanish in a way, but not in the same sense that someone who has lived in a Spanish-speaking country all their life and only speaks Spanish.
But cuylerotsuka probably just meant native speakers.
Writing dom is accepted in this course when translating, but unfortunately there's a bug that prevents us from accepting any kind of spelling variation at all in the listen and type exercises. Since this is a Swedish sentence, santeriduo must have had it as that type of task. More info in the FAQ that you can find under Swedish discussions.