No they wouldn’t stay lean. If you fed someone more calories than their body uses to maintain its current weight, they would get fat eventually. Regardless of whether they eat carbs, fat or protein.
You are right that different foods (macronutrients, more specifically) have different thermic effects and therefore require different amounts of energy to metabolise. Protein takes more energy to process than fat. This does not change my overall point.
The inventor of diet famously lost a contest for being too lean.