6 degrees renders large areas of the topics uninhabitable by humans. As in: extended periods where 100% humidity + outside temperature above human body temperature.

On any given day, absent technology, this will kill you in a matter of hours.

Indonesia (100+ million people) is a climate area that would experience this).

I'm sorry, but I need to question the validity of this claim. People have been living in very warm climates for a very long time.

I myself have been camping in the Mojave for several days at a time with temperatures high above 100 degrees Fahrenheit.

Are you implying that the combination of heat and humidity is the deadly combination? If so, can you provide some evidence?

High ambient humidity means no net evaporation of your sweat so no cooling effect of sweating. If the ambient temperature is above body temperature and humidity is high, you won't be able to cool yourself and will quickly overheat (keep in mind that you already generate 100 W resting). Your experience in the hot desert was different because it was very dry.