> When we talk about this document and how the judge are using it, what we are talking about is them using the scientific method as it is outlined in the document.

In this role 921 excels; it was written for the everyman (including the legal profession), and is thus accessible to those without the scientific background.
Call it Fire Investigation by paint-by-numbers.
So in a courtroom, where neither judge or jury necessarily understand the deep science behind it, it is a useful yardstick to measure the actions, steps and deductive processes of the FI by.
The FI (IF he is worth his salt) should have followed the same/similar paths (as described by 921) to his findings anyway; where he has deviated, he will have to substantiate it fully by base science (with all the challenges this poses in a courtroom).

But we should avoid the situation by which any document becomes the definitive reference instead of the base science upon which it is built :-
I can see some ridiculous situation arising in a courtroom near you soon, where
when asked to define a simple scientific principle like Work, Heat, Temperature,
your scientifically-sound explanation is rebuffed derisively with a counter-claim that
"but NFPA 921 says it is ..."

This happens in religions :
"But the Pope says..."
(i.e. canon-/ecclesiastical laws being quoted instead of the Word of God upon which they are based).
This leads to godless crusades, inquisitions and wars... (and Al-Qaeda, Boko Haram, ISIS etc.)