It's easy to explain your example with diabetics: very simply insulin is what allow muscle to use glucose, but also what stop the body from using its fat reserves. From your link:
> Desperate to stop its cells from starving, her body had released hormones that, in turn, released byproducts called ketones that turned her blood acidic. If she had not been rushed to the emergency room, she would have died.
This is similar to the effect of a ketogenic diet, when you avoid eating carbs to avoid insulin secretion, forcing the body to convert the fat reserves to provide glucose for the cells. Diabetics type 1 don't produce insulin, so they just skip insulin shoots to stay in that "mode" but it has obviously other serious health side-effects for them as discussed in that article.
Also individuals are not the same, there is a lot of variation in TDEE (genetics, health, weight, insulin sensitivity, metabolism, ...). But once you have figured the TDEE for an individual, calorie counting becomes very accurate. Just from the fitness community there is an overwhelming body of research and evidence: https://sci-fit.net/bulking-deficit-gaining/. If you are in calorie deficit, you lose weight. If you are in a surplus, you gain.