8%? No, you are clueless.
What most people, like you, fail to realize is that the more musclemass you carry, the higher amount of subcutaneous fat layer you can carry, and still be at lower bodyfat percentage.
Bodyfat is relative to the mass you carry. And Ferrigno obviously carries tons of mass. Probably 2x more mass than the avg gymrat, yes even at his lesser actor weight. With that he can have probably 1.5x thicker layer of subcutaneous fat and still be at lower bodyfat %.