They did a lot of the testing yes, but not all. They are not gods although I greatly admire Adams. Due to changes in the substances of a chemical compound it could very well be that their testingresults are no longer correct for todays used material.
5-10 minutes fixing seems overkill to me as it's not supposed to penetrate the paper itself but merely the chemical layer on top of it.
The materials they were using are still on the market. I use them.
Reducing fixing time was never recommended, but neither was extending it.
Not gods, yes, but they did know what they were talking about, and I trust that they knew what they were doing.
"Pictures are not incidental frills to a text; they are essences of our distinctive way of knowing." Stephen J. Gould