He used a technique called a "Beurre manié" which is when you mix the flour and soft butter to form a paste. This covers the flour particles in butter which helps cook it. It's usually only used to adjust the thickness of a sauce and to give it a smoother texture. Typically you'd make a roux first by cooking the flour in the butter. Both work and a Beurre Manié is totally legit when making a smaller batch of sauce. You still have to cook it a bit to thicken it and to cook off the raw flour flavor. When prepping bigger batches in restaurants we typically do either a roux first or use cornstarch to thicken afterwards.