There is no transfer of anything if the container is airtight, there is no osmosis or any other reaction possible between the contents of an airtight, non-permeable container and the atmosphere around it. I am only talking about airtight containers; if your snuff is in one it will not dry out or be cross contaminated, it can’t, there is no mechanism for that to happen within the laws of physics. Nothing out, nothing in - and it will keep for decades in perfect condition. Try it for yourself - take an airtight container and put a test sample of a moist snuff in there. Put it on a window ledge in full sunlight for a few weeks, submerge it in a bucket of water for a month or two, keep it next to some Gorgonzola for a spell. If the container is airtight your snuff will not have altered in any way. It’s not an academic question, just a fact.
If the container holding the snuff is not airtight that is an entirely different matter and in that circumstance a humidor would have an effect, depending on a whole lot of variables - but no more, and possibly less- depending on the humidor itself - than moving the snuff back to an airtight environment.