Well, in older RuneQuest, Moon and Light (and other runes) are swapped for the limited rune in question. Initiate into Yelmalio and you trade the "hot" bit of the rune for the benefits provided to a cultist of Yelmalio. It's not a separate rune; it's the same rune, so you just change your rune for the "limited" one. In the same way, We Are All Us principle meant that in earlier games you kindled your Moon rune (joined the Lunar Way) by trading your Fire or Change rune for Full Half Moon rune, your Earth or Life Rune for the Crescent Go Moon rune, and so forth. Each of these gave you access to powerful magics and elementals (Lunes) and set you on the path to Illumination. It also left you open to temptation, because the Chaos rune was always available to the Lunar initiate...
The real difference for me, I guess, is that Yelmalio is not Yelm. While I am glad we aren't in older Runequest, where there was a unique rune for everything, reducing runes to a mere handful reduces the distinction between the gods to name only. Yelmalio cultists don't wield fire magic, only light, and is the Little Sun as rewritten by Nysalor. This is why He is different: he is Light, not the Sun, and has a fraught relationship with the Solar Pantheon. The Sun Dome Temples rebelled against the Lunar Empire and seem primarily interested in their own survival and in converting the heathens.
Maybe this is nerdy? Idk, but they also write the Pure Horse god as "Yelm: the Sun Horse", which... I mean I certainly would never have written it that way. It's Yu-Kargzant (or Hyalor, I guess). There's really no reason to describe this god as Yelm.
I'm not trying to be a grognard here about lore, I swear.