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.

You are correct. One example that makes it obvious:

Take 100 calories of lettuce (which is quite a lot).

Now take 100 calories of oil (which is not much at all).

Combine the two into a salad, which contains 200 calories and 50% of calories from fat.

You learn something new every day.

Though I must say that "usually" doesn't really ring true to my ears. Perhaps it's the way nutrisionists talk, but is it really "usually" that way?

If someone presents a jug of a liquid and says "luckily for babies, liquid X is 50 percent fat", then I'd think that about half of the jug would be fat. Would most people where you come from think "half of the nutritional value of this liquid is made up from fat"? That's sounds impressively educated! Where do you come from?