How did we come to the hypothesis that the dwarfs burned out the dharr?
We know you can burn Dharr. We know there's a reduced level of ambient Dharr.
We know the ambient temperature is much higher than it should be.
We know the Dwarfs already have the infrastructure to gather large amounts of ambient magic in place.
We know something very fucky wucky is going on with the Beastmen.
Many of us suspect that they've been harnessed, controlled, bound, or socialized in some combination or manner.
The lack of ambient Dharr or a corrupted forest is strongly indicative that they are in some way modified to not behave like regular Beastmen. We know that whatever Cor-Dum is, he's not behaving like Cor-Dum, or Beastmen in general, have ever been noted to behave.
We know that the timelines of his disappearance do match up fairly closely with his arrival here slightly predating Botrek's departure South.
There is the possibility that it's an illusion, but to the best of my knowledge, no one has yet proposed a rational reason for the illusion to exist, as it would provide little to no benefit.
Therefore, we're operating under the assumption that they're genuinely Dum friendly Beastmen, which would solve a great many of the problems Dum, and holds in general, typically face.
It matches very well with Borek's caginess and reaction upon arrival.
It explains the shame.
It explains the insistence that the expedition not be an evacuation mission.
So far, it just seems the most coherent explanation. The devil is in the details though, and those are still up in the air.