They did research, you just used common sense. :-P
Of course you come from a long line of people who were selected on the basis of brains, not social class... (j/k)
But there is a serious point. How did England get to be so germanic? The obvious answer would be that a lot of germanic people invaded and overwhelmed the natives. Their research indicates otherwise, it indicates that a small group of settlers established a segregated society and outbred the locals. The fact that there were a small number of invaders rather than a large number is a piece of historical trivia that I wouldn't have guessed.
Now lest you think this is a stupid question, let's ask the follow-up question, why isn't France as germanic as England? After all France was invaded by germanic people as well, whose tendencies were the same as the ones who invaded England, but pretty much all that they left was their name.