Im going to go with there need to be a changing temperature. This is why the fogging recommendation is also mixed with the 18c night time temps at the same time. If cham,air, and fog are all the same temps, no change in moisture will occur. The cham will not dehydrate, the air will not increase in humidity, and the fog will not dissipate. But what if you have a constantly dropping temperature? This will cause warm bodies to release "fog" which also raises humidity. And during the morning dew stage, you have a cham that is colder than the air, breathing in a lot of condensation. You have warm humid air going into a cold chameleon, and condensing. I think is the time the chameleon is "hydrating". I dont think the chameleon is hydrating when its warmer than the air, i think its doing it when its colder than the air.
But that is assuming they are hydrating through the lungs or if they are hydrating through the sinuses/throat before it reaches the lungs. Kinda like humans warm and moisten air with the nose before it reaches the lungs, and also rehydrates the sinuses when they breath out.