Perhaps the difference is this: "beginner biking skills" vs. "beginner mountain biking skills". Most of the really easy, truly beginner skills are really just general biking skills, and I agree that for someone with no experience on a bike before, they are required.
However, for anyone who has spent some time on a bike, and is moving from path/flat smooth offroad riding to actual mountain biking, it's all about bike handling skills when things get technical.
I think the hill method for learning trackstands is counterproductive. One does not learn to control the bike, but relies on the hill to do it instead. The bike can easily be rocked back and forth with the body, locking the brakes to arrest the movement. This also more accurately simulates situations in trail riding, where a rock will stop a wheel dead, or a rider will brake until stopped before something hairy.
The best follow-up to trackstands would be hopping. This is just bouncing on the bike with the brakes on. You can use this to maintain balance, move around, etc.
K-turns are the best for learning not to bail every time your front wheel stops against a rock. Just let the bike bounce back and then ride around it. A confidence builder and a good beginner skill. An advanced rider will learn techniques to not let the rock stop the wheel.
Endo turns are a little more advanced... but it's a trick of the mind more than anything else. If you are rolling forward at a walking pace, lean most of your weight on the bars and lock the front brake... pick up your feet and the rear wheel will come up. Fear will prevent you from getting any real height at first, just practice. Next, turn the front wheel just before you "endo" and when the rear wheel comes up, move the bike sideways with your feet, pivoting on the front wheel. This skill is _essential_ trail riding techinque depending on where you ride. Some switchbacks just cannot be ridden through with a normal turn.
Anyways, it all depends on how and where you want to ride. These skills are probably not necessary in many places, and definitely required in others. I'm teaching my 9 year old son how to mountain bike this season. He's gone from barely being able to get down the urban path to negotiating fairly challenging trails in a couple months.