I'm not sure about this. Those kinds of temperature/humidity combos happen reasonably commonly in the American south and people aren't dropping dead, and weren't dropping dead before the advent of air conditioning.

Something's not being taken into account with this experiment, and I suspect it's heat adaptation. The study was done at Penn State in a relatively cool climate. I'd like to see this repeated with a variety of subjects who live in different climates...I think it would be more accurate.

(This is not to say we don't need to worry about this because we do, but...)