It’s probably dehumidifying the whole guitar, but slowly. Perhaps a better place to put it would be in the little accessory box in the case, with a few holes drilled in the lid.
Does it come with a humidity gauge? The real danger to your guitar is low humidity. The top wood in particular changes dimension a lot with humidity changes, and cracks ensue when the humidity gets much lower than the humidity at which the guitar was assembled. Received wisdom is that the guitar should be put together at around 45% relative humidity, as a good compromise.
There is a guitar case humidity control device available—I don’t have a link—that uses a salt that maintains humidity at a good level. It removes moisture when the humidity rises, and adds moisture when the humidity falls. It’s just the physical characteristic of the salt. A student of mine that runs a calibration lab uses the stuff to calibrate humidity gauges!