I think we have to differ between humidity of the air and humidity of the substrate. I entitle with you that the substrate shoudln't be wet. That have to make problems. But i think a normal (till 60 %) humidity of the air isn't harmful for the snakes. Rather I would say it's better than dry air. I've got to do some research to find the article where I read about the humidity (of the air...) and the problems, if she's to low.
Btw, the humidity in my fridge is about ~40 %. For my opinion, it's to low for a healthy hibernation. That's the reason why I put some moist kitchen paper over the substrate. So, the substrate is dry, but the humidity of the air ok.
A picture say more than my basic english words ;-)
Cyrill