: Ok, I feel it's time I actually threw some of my weight
: around here...
I don't think you can reason this out. What's considered "canon" in any lore is always based on concensus. Unless things changed a lot in my absence, there are still a lot of people who don't consider GURPS canon, not necessarily because there isn't good information in it, but because there's also a lot of bad information in it.
Where "good" and "bad" are rather subjective, and information fitting either category varies from person to person. :)
The other complaint I've heard about the Myth GURPS manual is the fact that very few people own it. I imagine it would be hard for non-owners to make an informed decision about whether to accept something that they've never seen.