A. Did the cop need to "brandish" his firearm...probably not... B. Was he allowed to...sure... ***The "unknown" had his hands in his pocket and was instructed to remove his hands from the pocket...and didnt do so. Furthermore, the cop's weapon was pointed down.
As long as the civilian kept the conversation present tense and forcing the officer to explain his actions based on immediate circumstances I was with him. He got a little personal and off script with a slightly heated tone and might have given the cop cause to act. Also that hand out of the pockets instruction seems like a tricky little trap if you do it too quickly. Like that guy in SC (?) who was reaching into his car to get his license, or pretty much every black guy shooting in the late '90s.
The cop may have been premature in pulling his gun, but the guy filming it was being a complete fool.