My intuition tells me that this argument about humidity as a universal factor of thermal discomfort smells like a trap.
It seems we forget too quickly that comfort is also a matter of local adaptation and available infrastructure, not just raw figures about air.
I have seen Montreal summers where humidity was terrible, but air-conditioned shopping centers and well-insulated houses made it quite bearable.
For a traveler, especially retired, the real challenge is not so much humidity but access to amenities that make it manageable, like finding a hotel with good air conditioning without breaking the bank.
It would be more convincing if the guide integrated the availability and cost of these adaptations instead of sticking to pure weather data, because the travel experience is much more nuanced than that.