Well, calories in, calories out is trivially true.

As the laws of thermodynamics, yes; applying it to fitness is intractably hard, and attempts to approximate caloric intake/expenditure for that purpose have all been intractably difficult or laughably bad. That's why it's not something that should be told to people just starting out. They'll inevitably use it incorrectly and get shocked when starvation causes their body's metabolic rate to plummet.