This really isn't the case. There is a concept called "defensive display" which is actually how most self defense with a firearm occurs.
An assailant is in the process of doing something warranting deadly force. The victim draws to defend themselves and the assailant sees the gun come out and disengages choosing to end the threat himself. You aren't requires to murder the fleeing/not a threat anymore person and you were still in reasonable fear for your life.
A small number of states have statutes that codify defensive display (you're explicitly allowed to warn someone you're armed, show them the gun, and shout "we will defend ourselves, stay back" before the threat even is imminent) but as a rule it's not always illegal to draw a gun and not shoot the person. It's more like a good guideline to avoid getting needlessly/maliciously prosecuted than a rule.
As I understand it the historic crime of brandishing is more akin to recklessly waiving a gun around so as to put the general public at harm, not really just having the gun visible or unholstered in view of the public. That is more of a colloquial definition. Honestly I don't know if it's actually a named crime anywhere these days. Boogie's actions do meet that definition of brandishing but as you can see his crime is AA, not brandishing.