Training is diffderent, for different situations
Keeping your finger off the trigger always works, no matter what gun you are using. It's that simple. No matter what the situation, no matter what happens, if your finger is not on the trigger (or in the trigger guard)
you will not pull the trigger unintentionally.
And, while there is a lot of overlap, there are differences between training in safe gun handling and training for a gun
fight.
I do get a bit of amusement from people talking about the "4 rules" or "Coopers 4 rules" etc., as if they were the only rules, or the only "important" ones. The aren't. There are a lot more than just 4. Cooper took the four that were most important to safety
and what he was trying to teach, and gave them to us in an easily remembered package.
Go take a hunter safety course, you'll find the "rest" of the rules for safe gun handling. All good, and sound rules, but, not always applicable to a gun fight.
For instance, "unload your gun before crossing a fence", is sound advice, and safe, but may not be such a good idea when a gunfight is imminent.
As to the full flap holster,
a nice relic, but not very practical for serious purposes today, as there are better options.
Americans have an almost ingrained belief that a "proper" holster is one that allows the gun to be drawn (and fired) in the easiest, most expeditious manner possible/practical. No one else does (unless they have adopted the American attitude). Even our GI flap holster works faster than other nations versions. Germany went through both world wars with full flap holsters that buckled shut. We had a holster, they had a luggage case.
Certainly there are better holsters for speed of the draw, and there are lots of circumstances where that is the paramount consideration. But there is nothing else that both allows you to have the gun on you, AND protects the gun from the elements as well as the flap holster.
A gunfight is the most serious thing you do with a handgun, clearly. But there are other things, that I consider "serious" (meaning not frivolous), as well. While it is good to be able to focus on a tree (and perhaps vitally so) there is still the rest of the forest out there.