That can't be true. You shouldn't sweat less in humid heat. If anything, you should sweat more in humid heat because your sweat isn't able to evaporate and cool you off.

Perhaps you could get dehydrated more easily in dry heat just because you don't notice the water loss as quickly since you stay drier longer. However, I would wager that if you hold air temp, exposure time, and activity level constant, then higher humidity would lead to greater water loss.