I think the majority view is that strenuous exercise does do a little bit of easily repaired damage to muscles, and that strengthening comes from the body's response to the increased load on the muscle fibers. As the muscles are loaded the body tends to build up those fibers. It's part of a 'use it or lose it' phenomenon that also applies to bone density, brain function and other aspects of aging.
I'm not an expert in sports physiology, but I seriously doubt that there is anything magical about whey as such. Many people do recommend consuming a mixture of protein and carbohydrates after exercise to replenish some of the carbohydrates the body used as well as to fuel the muscle development that needs to take place. IIRC the thinking is that: (1) Sustained strenuous exercise uses up much of the carbohydrate supply (glycogen) stored in muscles. The body wants to rebuild the muscle glycogen stores after exercise and if you don't eat some carbohydrate the body may break down protein to obtain building blocks required to create new glycogen; and (2) The muscles you exercised need a protein supply soon after exercise so they can begin the hypertrophy process required to increase muscle mass and build strength.
So, could whey help? Probably, but I suspect any other good, low fat protein source would work as well. In fact, because whey contains very low carbohydrate levels it may not even be the best choice for some exercises. Not all experts agree on what the proper ratio of protein and carbohydrate should be post exercise and it is even possible that the optimum ratio depends on the nature, duration and intensity of exercise, but most people believe you need both carbs and protein within about 2 hours after strenuous exercise. Turkey sandwich anyone? (hold the mayo). Now, if you are weight lifting you may not need a lot of carbohydrate after exercise because weight lifting probably does not burn up a lot of glycogen. OTOH if you are riding a bicycle you probably do need to replace a moderate amount of carbohydrate in addition to the protein. Most people should do both resistance (weight lifting) and aerobic (running, walking, cycling, swimming, ...) exercise.
Does this sort of post-exercise supplementation interfere with weight loss? Maybe, maybe not. Quantity matters. You don't need a huge meal after exercise, and it is easy to overeat. Whatever you eat post exercise counts towards your daily allowance, so if you eat a big meal or take a lot of protein supplement, ... then you have to cut back a bit elsewhere in the day's intake.
Still, as Evie observed, the exercise can contribute to long term weight loss even if you do not initially lose because the muscle you are forming is metabolically active. More muscle = faster metabolism and that can contribute to weight loss over the long haul.
The trick is that when you increase your exercise you have to increase your intake by an amount less than the calories you burn during exercise.
Personally, I advise patients to pay less attention to weight. I want them to focus on exercise and proper diet. If you work on those things the weight will usually take care of itself. The thing is that you cannot directly control your weight. What you can control are diet and exercise so you need to focus on those, not the thing you can't control.