I served this one up on a platter for someone to take but I think lkozma's reasoning is more plausible. I don't think ErrantX's comment was based in ethnocentrism. It's just a way of describing a non-regional accent in the US and apparently the UK.
People that think they have this accent or that someone else has this accent will often refer to it as "no accent". It's not an ethnocentric viewpoint. Now, I definitely think the average person is not aware of how distinctive their accent really is and mistakenly think they have a non-regional one. But I do think it exists.
Just to give an example off the top of my head: take Conan O'Brien. He's from Boston, went to college in Boston but does he have a Boston accent? Maybe you could pick it out of his voice, but the average person probably wouldn't and he certainly doesn't have a strong one.
I then spent 2 years in Boston. And now 2 years in Toronto. My accent is completely mangled and jumps all over the place (ie, I might sound vaguely North American, but I know that you there's a 'h' in 'herbs').