I don't think that Yelm, as a great god of his element, is the source of
other solar powers, but the idea that a great god as the most prominent
god of its type has a deeper ghasp of those truths associated with its
rune. It is only reasonable that Orlanth, the storm that is most
universally recognized by theists, has more universal associations with
the element. However Orlanth isn't the source of storm and Yelm isn't the
source of fire. They both are the current holders of those runes, just as
the good old GL sages observed. So... it could be said that Yelm is
closest to the solar secrets and his culture understands this to mean
Aether.
Why couldn't there be different markes of great godhood? In the past
holding a rune sufficed, then at one time you needed a certain number if
worshipers (divine mechanics disagree with this one) and nowadays a sure
way to qualify is to be the main god of an entire culture. I'm sure that a
god needs to be pretty significant in order to be great, but that can come
in many forms.
Olli Kantola
End of The Glorantha Digest V8 #510