It's a turn of phrase. The lifting essentially damages the muscle tissue - if you eat in a caloric surplus and plenty of protein to create a positive nitrogen balance, your body will have the resources to repair the muscle tissue and make it bigger and stronger to better deal with that stress next time. If you don't provide those resources, you can't build muscle. So what you eat is just as important as how you train.
If you go gym, and eat kebAbs every day, what is the outcome you think? (some would say it's fiine as you work it off, but everyday defeats the objective)
If you go gym and eat healthy & high protein food, what's the outcome?
If the macronutrients were more or less the same each day, the result would be the same in terms of body composition. The last option would just be healthier.