The "Travel Guide Seasons and Climates" assumes that the impact of humidity is a fixed property of the climate, but my intuition tells me that this is far too simplistic.
It smells like a shortcut for guides who want easy classifications to sell to travelers.
My experience tells me that a good air-conditioned hotel in Singapore makes a humid summer quite manageable, unlike a tent without air conditioning in Quebec.
Infrastructures change everything: the same level of humidity does not have the same impact depending on the context.
What would confirm this would be to see a study comparing the perceived comfort in similar humidity conditions but with very different levels of infrastructure.