That so-called instructor doesn't have a clue as to what he is talking about. Suggesting single-action in a close quarters defensive situation can be a recipe for disaster. I was a Police Firearms Instructor for over 20 years and every agency I knew of trained personnel to always use double-action in such encounters. A deadly force incident creates sensory overload, (maximum pucker factor), even among highly trained individuals, causing a large decrease in the fine motor skills necessary to draw, cock single-action , and fire accurately on a target. Even if accomplished a weapon with the hammer cocked back in single-action requires very little effort to fire and may do so at the worst time. One may trip or stumble resulting in an unintentional discharge resulting in accidental injury to a fellow officer or other companion or an uninvolved bystander. The result can result in the shooter suddenly becoming a defendant in a criminal/civil action.
With sufficient practice double-action can be very accurate and controllable and considerably less likely to result in an untintentional discharge. It has the benefit of requiring fewer manipulations and has the same trigger stroke for all shots (KISS). Yeah, I know there are some individuals who are highly efficient firing in single-action but for most of us it just isn't so. Law Enforcement agencies did train in single-action shooting for many years in the past, but all I am aware of now require double-action firing with revolvers and it is for the reasons mentioned above. They learned the hard way.