Ah, once again opening the can of, "What is your response in the face of a drawn gun," COW.
While that's true the facts are that in a majority of civilian armed encounters and also off duty plainclothes LE, the defender is facing an attacker who has already drawn his or her gun. No matter how aware of your surroundings you are, you are never going to be able to outdraw some attackers in some situations, unless possibly you carry your handgun in your hand all the time, like a armored car guard carrying a bag of money. If you draw down on or in some cases or jurisdictions even display a gun in your hand if you encounter a suspicious person or persons, even if you don't point it at them, it could possibly get you arrested if you're wrong, and they turn out not be armed at the time the cops are involved.
I wouldn't be surprised if this video ends up on the Active self protection series of videos, where the video is analyzed by the host. I have no idea what he will say, but I know what happened in a previous incident. This video at 2:21 to 2:22 is almost exactly like a defensive shooting involving an off duty cop in plainclothes decades ago, including the position of the attacker and defender relative to the car. In that case, instead of walking toward the attacker from the back of the car, he stopped, took a step back, and then moved right, so that the body of the vehicle was between him and the attacker. This gave him the opportunity to draw his gun without being seen by the attacker, and forced the attacker to move in toward him in order to get a good shot. When the attacker did, as soon as he cleared the car, he was shot before he could react to seeing the cop, because the cop was already prepared to shoot the attacker had the attacker advanced, which he did.
In that old case I don't recall the cop was attacked by any accomplices like the video in the OP, but those juveniles in the video didn't come in from behind until after she was confronted by the first attacker anyway. If she had been armed and positioned herself in a similar manner, she could've dealt with a threat from her left side also.