Usually when people state that a food is X% fat, they are talking about its nutritional information values, and your link does in fact confirm that about 50% of the calories derived from breast milk come from fat:

"Human milk contains 0.8% to 0.9% protein, 3% to 5% fat, 6.9% to 7.2% carbohydrates"

Excluding alcohol, essentially 100% of the calories of any food come from those three macronutrients-- 4Kcal per gram of protein or carbohydrate and 9Kcal per gram of fat.

9(3 to 5) / (9(3 to 5)+4((.8 to .9) + (6.9 to 7.2))) = 45% to 59%

Assuming the data in the wiki you linked to is correct, 45% to 59% of breast milk is fat, in nutritionist lingo.