I'm not sure if I have ever encountered such an extreme definition of descriptivism. I am a descriptivist, but I still believe that you can sensibly state that

I'm not sure if I have ever encountered such an extreme definition of descriptivism. I am a descriptivist, but I still believe that you can sensibly state that some otherwise intelligent native speakers are "wrong" in their use of the language and certainly not everyone deserves the title of master; it is still sensible to talk about different levels of native language mastery. Consider that there are certain types of grammatical errors that only native speakers make due to regional idiomatic usage. Those people will agree that their usage is incorrect (they were likely taught so in school) and other native speakers from other regions may struggle to understand them. Are they truly a master of the language, or are they a master of their dialect? Can they switch off their dialect and speak flawlessly without major effort? Then consider that there are more non-native speakers of the English language on Earth than there are native speakers. Usage of tense, articles, prepositions, and so on is a cultural artifact. A group of non-native speakers can in some case carry on a conversation fluently that a native speaker will struggle to understand because of frequent usage of uncommon grammatical artifacts which they are not comfortable with. Is the struggling native speaker truly a master if they take so long to parse rare correct grammar?