Debunking myths and legends in language learning is a Sisyphus-like task, and not a day passes by that I have to bite my tongue and hold my horses when I hear someone telling me harebrained idiocies about the nature of language, on how best to acquire a foreign language, and what to do to speed up the process, mainly as a result of legendary myths ingrained in the average human brain.
Native speakers are authorities on their language, regardless of their cultural background and formal education. In my opinion this myth stems from the fact that all monolinguists are cut from the same cloth: they believe they are masters of their tongue simply because they learnt it from birth and it is the only one they speak. They take it for granted that a Rumanian, for instance, speaks Rumanian perfectly just because he was born in Rumania and is a native. I say that a native speaker with a Ph.D. in English does not have the same proficiency as a High School dropout.