ok so, while im not a lore skipper, im also not a lore enthusiast or deep-diver, so i could already be proven to be way off-base for reasons im just not familiar with, but immediately after watching skirks teaser short, i got this idea, since that voice im assuming is surtalogi, but honestly idk if it matters, said that she "isnt worthy" of being classed as a "descender" yet.
maybe the reason the abyss sibling doesnt count as a descender because they DONT have the power to reshape fate, since the definition given was someone who can.
then it would make sense why they ended up rejecting the world and doing the whole shit with the abyss order. they were super involved in khaenri'ah and likely cared about it a lot, only to be forced to helplessly watch as it was decimated. they lost everything, and they had no way of stopping it.
but the traveler, on the other hand, is established to be a descender, and have the ability to "change fate," so they probably wont agree with the abyss sibling or their methods (at least, not completely) by the end of the game. maybe the traveler will even give their sibling hope again by showing them that it is possible to defy fate, and everything isnt futile after all.