Drunk guy threatening? Open Carry? No one was hurt? Southern Rock? All seems fine to me
Had he done anything you would have the law on your side. You attempted to defuse, you had the right to be where you were, you did not instigate, and he was chemically imbalanced.
Some people will say that you could have avoided the situation entirely by not OCing. To that hypothetical I would respond "what if the drunk had been planning to rob them but saw the gun and decided to back off?".
One could make a comment on your situational awareness when you are working on a car, though, especially if you are under it. It's hard to see, people can sneak up on you easily. Glad to hear no one was hurt.