it makes sense. Pneumatic tyres work by stopping the moving object having to work against gravity to rise over bums in a surface. When your bumps are on a large scale, e.g. a rocky trail surface, you need a lot of "give" in the tyre to squash enough to avoid working against gravity, hence low pressure/high volume.
It seems "counter intuitive" because on-road , where rolling resistance is more significant, it feels like that. In this case the bumps are a lot smaller and you need a lot less movement in the tyre to iron out the bumps. However in this case, there are other significant effects such as shear wich are not that well understood, and make the rolling resistance lower when it's wet. There was some research on road bikes which said you want the tyre to sag 15% when you sit on the bike, and that's the best balance between low contact area reducing friction and enough give to get over bumps. I suspect this does depend on the surface though.
That could all be rubbish but is said with confidence (that must count for something?), and there is research to back most of it up….