Sure; from a purely mechanical standpoint, if you burn more calories than you consume, you have to lose weight (note that whether the loss is muscle or fat is not specified). However, it isn’t that simple in practice. For example, if your body goes into a conservation mode, you simply don’t have the energy to exercise at a high level.
Huh? You're here arguing against calories in vs. calories out and you need someone to tell you whether the human body burns fat or muscle tissue first when it needs energy? Really?
For example, if your body goes into a conservation mode, you simply don’t have the energy to exercise at a high level.
This is just more nonsense. Anyone who has ever lost weight by restricting calories and exercising has worked through what you call "conservation" mode. You act like no one has ever lost weigh by eating less and exercising more. In what world is that the case?
Right, because if you go the starvation route metabolism slows and the rate of burning calories slows down. Weight loss can come from muscle loss, bone loss, fat loss, or water loss. I am talking about losing fat. Most people are not interested in losing muscle, bone, or water.