You are correct -- the lactose is a polymer of glucose and galactose while sucrose (table sugar) is a polymer of glucose and fructose.
The importance of the statement isn't to learn nutrition trivia but to recognize that milk sugar does not contain fructose. In high (I recognize the vagueness of that adjective) amounts of fructose seems to have a negative affect on overall 'metabolism'.
The practical issue with milk sugar is that, as mentioned by Merkaba, is relatively 'empty', especially in regard to cutting.
Nonetheless, it has been a staple for bulking for decades. However, the milk of today is not the same as yesterday. That's a completely different conversation that leads away from the point that nutritionally dense food is especially important during cutting and even though one can get away with milk sugar from a calorie in/calorie out standpoint, it may have negative influences on satiety, especially when one can select vegetables for a primary carb source.