Exercise Physiologist here.
Being that he is in the NFL and its all about peak performance and measurement I have to assume the BF% reading was from a DEXA or DXA machine (they essentially x-ray you and it gives numbers based off density, it really doesn't get anymore accurate).
Healthy ranges for BF% depend on gender and age. For an elite level athlete 6-10% would be considered pretty normal, especially when in season. Usually not at the lineman positions, but for Donald's sake I'd be afraid to call him usual or normal.
For men generally speaking you or I walking around daily at a healthy level you'd love to see in the 12-16% range, 20% would be on the higher but still average side. Over 30% is where we see a increase in metabolic and cardiovascular risk factors and disease. Perfectly conceivable Donald could be rolling in at 10% if he took care of himself in the offseason. Generally 2-4% is considered essential fats that you would only see withered away by extreme starvation.
For example I tend to measure in around 4% myself, just the point my body likes to be at. I am no world class athlete nor am I on an extreme workout program. Thats just where my body likes to sit at on a normalized diet with regular exercise (in fact, I really don't ever do cardio....waste of time). No I don't "cut". No I don't use any steroids or diuretics. No drugs actually, unless you consider beer a diuretic haha.
Body fat would make no man better at playing football. Its just weight, it doesn't prevent injury. It doesn't make him quicker. It doesn't allow him to be stronger. It doesn't increase explosiveness or give him extra energy in the 4th Q with the clock running down. Its simply reserve energy stores that take quite a long time to be mobilized.