This particular item, in my opinion, is a great example of something that is not a matter of who is right and wrong as much as what works for one rider versus another, depending on that riders bike and their physical build and flexibility.
Different bikes have different rider handlebar heights and distance from the seat, different shaped tanks, different rearset heights and configurations, etc. and that all impacts how the rider can hang on, and hang off. Even if you just narrow it down to sportbikes, you can look at a Ducati gas tank versus a Yamaha gas tank and see that rider lock on will not be the same from one to the other. And of course, a 6'3" 180 lb rider would fit on a bike differently than a 5'1" 180 lb rider.
You can go to any track day and see LOTS of riders hanging their butt WAYY off the seat, even riders who are riding at a slow pace in the beginner group. Very often you will ALSO see those riders propping themselves up with their inside arm, and/or crossing their head and upper body BACK over the tank to the other side, so they really aren't shifting any weight to the inside after all. (OK, gallery, what is wrong with propping yourself up with the inside arm?). Some riders are strong enough and flexible enough (and tall enough!) to find a position where they hang off more than half their butt, without causing any unwanted bar input, unstable lower body lock, or excess fatigue - but for MOST riders, half a butt cheek is a much better starting point to create a stable, functional and effective body position.
At the school we have a great off track exercise where we put a rider on a bike and work with them one-on-one to find a body position that works for them, along with educating them along the way about what is important about body position - what is the point of hanging off, how to do it (if desired), and how to get a good, comfortable, solid position that works, and then practice it. Just like you say above - knowing not only what to do, but also understanding why.