I agree that fat loss and weight loss are often used to mean the same even though we, as professionals, understand the difference.
Particularly when dealing with women, though, there is another powerful way to communicate that concept which is dress size. It is not coincidence that so many ads will feature somebody saying: “I went from a size 18 to a size 8.” It is a bit more tangible.
Fat loss in particular, while more accurate, is difficult for a layperson to relate to. We can measure it with body composition but the client by him/herself can only see numbers as in weight, dress size and inches.