The left arm going outwards is just a product of the shoulders opening and helping your body come around gaining torque. As your body comes around pulling your arm inwards helps you gain extra speed as you come around.
Also, be careful when watching "old" tennis. There are a lot of other contributing factors than just pure kinetics. No jumping on the serves were a rule. Grunting was discouraged. I could totally see flailing arms being ungentlemanly. Also, 2HBH's were "weak" and "unusual" back then.