Sorry to bring up a new argument in the conclusion... But the first debate I won in high school was because I had a suprise argument.
The Delta-X Criterion, which I use for my own speed determination (so I subscribe to it but I want to explain something)... is based on the agreement that 0.3 Gradient is the appropriate speed point... Based on the study of The First Excellent Print.
Zone System speeds are NOT based on the study of the The First Excellent Print. So there is no reason except coincidence that the speeds even closely relate.
The First Excellent Print is based on people who viewed prints and pointed to the ones they liked. Built into this study is a Standard Observer's Opinion that "some" of the shadow detail is not important.
The biggest difference between this and the Zone System is... You will never hear anyone who uses the Zone System saying that shadows are not important.