Brake horsepower is the power at the output of the engine. It's called that because it is measured with a brake dynamometer. Wheel horsepower or wheel brake horsepower (same thing) is measured after mechanical losses (at the wheels).

But "they aren't looking, why don't they just.." really rubs me the wrong way. Surely these people are experts in their own field and know what's available. After all, they are ordering trucks from a vendor, not designing trucks to their own unique requirements. Plus these Mack trucks indeed already have regenerative braking.