You seem to be missing the point that standing stationary with no shade in 100+ weather will make some people sweat, even if in shape. Its a natural response, and thats before you consider your attire.
Again people can bring a change of clothes, but Ive worked places that require a suit year round, or at least no shorts. Biking around in pants in 100f heat is hardly fun.
For those that need to be in a suit, I don't think cycling would work at many times of the year. But if you just need to do some grocery shopping, then a bike with a couple of pannier bags works well.
As I said, cycling is actually more pleasant than staying stationary.
If you want to argue that cycling isn't suitable in an environment in which humans should never be outside and only transfer from one air-conditioned environment to another then fine, sure, but my god, what a dystopia in which no human should live. Time to encourage migration to Siberia rather than cars.
Well, I would argue that 38C and above are hostile temperatures to humans and people probably shouldn't go outside too much those days hahaha, but it also depends on humidity. I do think cycling is suitable even in those places, and I personally cycle under those circumstances, but there should be a temperature controlled public transport option available as well.
I'd measure it relatively against the amount they sweat at rest/walking and sure, some people sweat even then.