That is very incorrect. The hand signals are universal between bicycles, motorcycles, and cars, so you use the arm that you could stick out the window in a car. In places with left hand drive, you'd use your left arm. To indicate a left turn, you stick your arm straight out to the side. To indicate a right turn, you stick your arm out, but bend your elbow 90 degrees so your forearm is pointing straight up.
This is a Dutch artist, and a Dutch landscape. The Netherlands is a right-side driving country.
I guess your comment is true for car drivers sticking their arm out of a window, but for cyclists it really doesn't matter. You can use both arms to indicate direction.