You can dress a pig in a sweatsuit, but that doesn’t make him an Olympic athlete.
And you can attend a ten-day defensive firearms course, but that doesn’t make you a gunfighter. It may make you adept in the fields of weapon manipulation and accuracy with a specific firearm. You may even walk out of there with a modicum of strategy and tactics—but it’s not the same as retaliatory defensive shooting when under attack for real.
Defensive shooting, by nature, is retaliatory. If you have the opportunity to smoke somebody’s hindquarters before he can react to your action, you are not responding defensively. You are initiating an action, even though it may be warranted, and this, by definition, is not an immediate defensive response to a deadly force threat.