This isn't mastery of the English language, he's mastered it fine. This is mastery of American working culture, which is optional. The reason why he grates Americans so much is because he's direct about his feedback, which Americans take as anger.
To me (a Greek), what he says is fine. He never gets personal, he talks about the arguments/points/issues, not the person. "complete garbage arguments" is a judgement of the arguments, not of the person, and it's fine to tell someone their arguments are garbage, if that's what you think about them.
Americans/Canadians/the English will interpret that as anger, but, in other cultures, it's just voicing one's opinion.
To me (a Greek), what he says is fine. He never gets personal, he talks about the arguments/points/issues, not the person. "complete garbage arguments" is a judgement of the arguments, not of the person, and it's fine to tell someone their arguments are garbage, if that's what you think about them.
Americans/Canadians/the English will interpret that as anger, but, in other cultures, it's just voicing one's opinion.