Being a good Fire Engineer, does not make you a good Fire Investigator;
NFPA 921 would guide a FE to be a more competent FI, and NAFE would be naive to ignore such a thorough, systematic source of sound advice.
That said, NFPA921, inasmuch as it's contents are sound and correct (and, in my opinion, extraordinarily-thoroughly reviewed), is hardly technically challenging;
it is, after all, written to be accessible to all (even lawyers
).
If a heart specialist is presented with a guide on nursing cardiac patients
he would, despite probably being short on some nursing skills and experience himself,
be incline to treat it as a guide (particularly if it's own preamble stresses that it is a guide).
When you blindly follow the text without wondering where it came from, it is a religion, not a science.