The Karaz-Ankors Waystones work differently from the normal, Ulthuan-centric network. According to Boney, only the Waystones in the Karaks actually collect magic, the Waystones between the Karaks do not have this feature, they only direct the flow of power between Karaks. This makes them much harder to find for potential bad actors, increasing the security of this vital part of Dwarven infrastructure. Since there are no other Karaks north of Vlag, all the power that is used to keep Vlag trapped in the Warp has to have been collected in Karak Dum. Which makes sense, the Dum Waystone is situated in an environment that is operating under constant Storm of Magic conditions, so there is lots of magic to siphon.
Even if it can't power the Rune of Valaya of the Hold due to being cut off from KaK (and i wouldn't bet on that, Dum Runepriests were presumably pretty hardcore and they had a lot of time to prepare for being overtaken by the Chaos Wastes), at least it should take away so much magic that Chaos can't freely spawn Daemon armies inside of the Hold, or do any other reality warping bullshit common to the Chaos Wastes, like massive time dilation or spontaneously turning walls into quivering piles of bleeding flesh. That's a big boost to defensibility.