Neat tactics there, though more questions come up about timing, aging, and causes. For their area to go from a lush deciduous environment to ice world would normally take at least centuries to millennia in a standard cycle on Earth. Given their superhuman capabilities and how the priestess still looks ageless despite being older, it's been hard to tell exactly what the lifespan of that tribe is though it wouldn't be surprising if it was a lot longer than baseline humans. But unless the pseudohumans are also special in that regard (or that experimented one was), it must have been way less than a century. That's a really rapid change. There could be some weird exoplanet climatology or orbital cycles or something going on, or maybe there was some actual disruptive event, a major asteroid strike or supervolcano or the like (on the far side of the world they wouldn't necessarily directly see any such thing, they'd just feel the environmental effects).
On the other hand volcanism might offer them a reprieve too.