'Calories in, calories out' has been pretty widely debunked - at best it's part of the story. More important is meal timing (i.e. fasting), resting insulin levels, the source of those calories, etc.
Not OP, but the general logic behind "calories in = calories out" is obviously fine. However, just knowing this doesn't really tell you anything about the best way to go about decreasing "calories in" or increasing "calories out" in a way that will result in meaningful fat loss and improved health. For example, it is plausible that someone could reduce "calories in" by eating less and their body could make up the difference by reducing "calories out" via lower BMR instead of having "calories out" remain constant with the difference being made up by burning fat stores. Maybe some fat loss will occur initially at the expense of being more tired and hungry but this change eventually becomes unsustainable/unhealthy if they aren't eating the right foods.
On the flipside, you can increase calories out by exercising but you could end up starving and/or exhausted as your body tries to compensate for the increased activity, depending on what is going on in your body hormonally. Or you just have no energy to spare for exercise as it is right now, and have a very hard time getting in to an exercise routine. All this is to say that "calories in = calories out" is all fine, but these variables are not independent. Having some knowledge of how and when the human body burns fat and when it stores fat goes a long way in coming up with a strategy to lose weight. This is where these trends of low carb diets and (intermittent) fasting have originated from.
You nerd to get a new Dr. or nutritionist if that's what they told you. If you just got that from rando internet bro science you should talk to a Dr. or nutritionist or try to back your statement up with some clinical research.
