I find this interesting because we all obviously know this, but does the average NFL fan? They look at buffalo, see 40+ points and the best offense in football and do they really look at McD and think defense?
The HC position is so much more than what side of the ball you have experience with. The intricate knowledge and experience needed at this level often makes coordinators masters of both sides of the ball.
Once a guy becomes an HC, he no longer, in theory, has a side of the ball. He just wants to win football games