'Calories in vs calories out' is more meme than myth. It's both true and nearly worthless to aid anyone in weight loss because it's not the thing anyone is confounded by.
“Calories in vs calories out” is a restatement of the second law of thermodynamics and must be true.
Unfortunately calories out is excessively variable, as are hunger and energy, which affect calories in.
Perhaps from a purely academic sense but in practice it's pretty reliable.