Well, they certainly broke the 1st commandment of safe travel mentioned in that book.
"Thou shalt go one at a time - and leave someone in a safe spot to do the rescue."
There's no question of "if" mistakes were made, it's just about finding out the details to find out WHAT mistakes were made so that others can learn from them. If someone triggers an avalanche and gets caught, then mistakes were made. The goal is to not get caught in an avalanche. After you're caught, then it's up to luck and your buddies to save you. When your buddies are also caught, then it's completely up to luck. I don't like relying on luck.
I know most skier deaths in Colorado are due to blunt force trauma. I.E., hitting a tree, lift tower, another person, etc.
As far as the patrollers go. That just sucks. No doubt that mistakes were made. The reasoning for those mistakes is what is going to tell the story. I will say that avalanche mitigation work and backcountry protocols in general are fairly new to Japan. Up until recently, they only worried about roads and well controlled on piste terrain. Wild snow was not part of their management game...