Shouldn't that be the other way around? You can make the extrapolation from the (more accurate) reading of the wood to the materials around it, but not from other materials back to wood. If the resistance of wet drywall is equivalent to that of wood at 20% moisture, I'd think that the drywall is a lot wetter? And using the device on a steel beam might show some insane percentage when we know steel does not absorb water.

Someone should make a specialised tools community like Reddit's. Would be a good fit there