I am assuming that post was about AC, as flying out of Denver in the summer can get pretty hot, especially with Boutique's conservative use of the aircraft's AC and janky ground AC carts.
I have seen temps inside the cabin as high as 48º C, and after an hour long flight with functional AC we were only able to get it down to 35ºC.
With no AC at all, it could absolutely be unsafe to fly in that kind of heat.