It's not about his skill at speaking the language, it's about understanding what is appropriate to say and what is not. Just because someone is technically skilled at something doesn't mean they know the best way to use it. That applies to everything including language and tennis.
Well, out of the two of us, you're the one whose mother tongue is English, so I sound silly arguing otherwise. But I doubt many people actually believe that and agree with you. I think Roger knows very well what he is saying. He is a truly intelligent man.