I'm guessing that's based on personal experience and not hard numbers. Based on my own personal experience, I will disagree with "a lot." It really depends on what fandom you're in. I have been in fandoms where a good majority of Brits didn't give a shit that they were writing for an American show. Conversely, I've been in others where Americans really did their research and were diligent about "sounding" British. As with everything YMMV, but don't try to make a conclusive statement without knowing that you're being a twat by making it.