> Your diet doesn’t matter if in the end you consume more calories than expended- you will gain weight.
This isn't true though, it's not the amount of calories you consume but the amount of calories your body actually accesses out of those consumed.
If most of the calories you consume simply pass through or get consumed by the parasites living in your large intestine, calories consumed can exceed calories expended by you while still losing weight.
The researcher who discovered the general role a pancreas plays in weight gain back in the 1800s had dogs living normal lives after removing their pancreases. Except no matter how much food they consumed, they continuously lost weight, and eventually died as if they'd starved to death. He discovered their urine was sweet after seeing flies were more attracted to their urine than the normal; without pancreas-created insulin the fat cells wouldn't store the energy, it was leaving the system in the urine.
This illustrates the role hormones play in energy storage/release. Everyone's hormone levels are different, and how they respond to different foods varies, it's all very subjective, sex, and age-dependent. So even within those calories accessed via the GI tract, there's hormonal variability in what gets stored vs. used vs. excreted.
Calories consumed only sets an upper bound; if you consume less calories than expended, you will lose weight. It's not a symmetric relationship like your statement assumes.