Other than the fact that Schwarzenegger is Austrian by birth and emigrated when he was 21, I suppose that he's American enough.
(At least that's what the Republicans hope.)
When you really think about it, doing a really good job as a villain requires a great deal more skill as an actor than playing a hero. Films tend to explore the mind and psyche of the villain more so than the hero. The hero is boring. Some innocent fresh faced whatever who doesn't have to have much of a personality. He just needs to be charismatic but that does not equate to complex character.
The article brings up a few things about "British" people that seem appropriate for villains but there's more.
Frankly, there is a way for British actors to just be darn right more menacing than most American ones. A lot has to do with the ability to portray a much cooler and methodical demeanor. The accent certainly helps with that (along with a touch of arrogance if done right). But that's not to say that British actors are any less capable of portraying truly insane, frothing at the mouth types of villains. They also do a better job at the snivelling cowardly types of villains as well.
Villains tend to be much more complex characters in movies and frankly British actors are better at it.
A lot has to do with many British actors having a great deal more formal training in classical drama. Theatre relies on complexity of a character and believable portrayal of emotions rather than gee whiz special effects and gratuitous violence and sex.
There are sub-plots and subterfuge in a lot of drama that most US actors are never really exposed to while it's commonplace for British actors.
I look at the past 2 generations of dramatic actors from the US and England and the majority of truly talented actors are British.
If you want to boil it down to one word: Shakespeare.
