"Dia adalah seorang juru masak."
Terjemahan:He is a cook.
You're right, I forgot to explain that here. Boss of the cooks = it means a professional context.
There is usually several cooks in a profesional kitchen, and in this case, the chief is only the boss of the cooks. Profesional kitchen are very hierarchized. You can't have several chiefs. You're right, some restaurant like pizzeria can have only a cook, the pizzaiolo. In this case, profesional cook = chef.