Until he develops tendonitis and is unable to play, that technique has enough credibility to support the statement that the standard technique today is having the thumb as a pivot behind the neck. You are right, over time the standard of the bass playing technique might change, but you can't teach that until you prove yourself, there is nothing to back it up.
You can't teach what you think is right if only a handful of people in the bass community thinks its right.
Im not telling you that you are wrong, but if you want to teach something like technique, there must be a proven method. How would you feel if someone was teaching you how to build a car and they told you "if it has 4 wheels and you feel that this car is safe for you then go ahead and make it" ... you won't be selling many cars.
Again, this comes from the teaching pov, there has to be a concrete idea to teach technique. If there is something we can take from the history of string instruments it would be from the classical guitar technique. Its the same technique for ages.
Now, you are right when you say that everyone is different. But in that sense its not ok to tell them to put the thumb anywhere they want, they could be doing more damage to their hands and be very bad in the long run.
Opposable thumbs are what separates us from most animals, why not use it to your advantage.
And there is the first flaw the history of stringed instruments.
The issue on the history is based on a finger board not a fret board, so comparisons are flawed.
As for up-right bass the issue is one of plane.
The up-right bass as the name implies is a vertical plane on a finger board, a bass guitar is a horizontal plane on a fret board.
Add to this the construction values that means an up-right bass needs size to maintain its sound and the stability of the neck joint, then the neck thickness is based on the materials available at the time to the tension of the strings to the overall size of the instrument.
Take into consideration that the up-right bass is designed on looks rather than any ergonomic design and again the techniques developed are based on dealing with all these issues.
A modern bass guitar has none of these issues, so any of the so called correct techniques of an up-right bass cannot apply.
I completely understand the idea that they can, but the reality is they do not. Our body and it's mechanics work better in a vertical plane than a horizontal one.
It's not about teaching what is right, it's about adapting what can be done to be the most efficient.
What is efficient for one is not efficient for another, we know that, this is not up for debate. The amount of injuries that present themselves with the same symptoms between musicians and none musicians are comparable to sportsmen and non-sportsmen.
I know players that have "tennis elbow" and have never played tennis in their lives.
The issue of correct technique for the vast majority of players is about comfort first and foremost.....after that we wait and see what presents itself then deal with that.
As you say with the Matt Garrison until something happens it is the proven application of technique for him...and that technique came about because it was the only way he could cope with the demands of what he wanted to play, not because it seemed like a good idea.