That’s true re pronunciation. The discussion had to do with proper grammar.
There is more to the "King's English" than just grammar. It also includes pronunciation and diction.
If I speak "King's English," using the proper grammar but with an American or Australian accent and/or pronunciation, am I really speaking the "King's English?"
WRT the phrase in question as published, it is more likely British slang than proper "King's English" grammar.