> Combined with power outages, as happens in the book, everyone living under such a heat wave will likely die.
That's an exaggeration. Extreme heat (including heat waves that exceed this threshold) happen semi-regularly, and the death rate when they occur is nowhere near 100%. More like 0.1% or less.
Electrically-powered air conditioning is not the only way to cool a human body.
Simply escaping to an underground or substantially enclosed area is sufficient for periods of up to a few days, or even weeks depending on the thickness of the walls. Car parks, cellars, caves, etc... all retain a relatively constant temperature irrespective of temporary swings of the air temperature outside.
Similarly, immersing oneself in a body of water below about 36 C is also sufficient, and that's typically available even if the local air temperature and humidity exceeds safe levels. E.g.: Rivers "carry" the cold down from mountains, and are safe havens during temperature extremes. Even the local pool is sufficient, because water has such a massive thermal capacity that it won't heat up anywhere near as fast as the air around it.
At times like this you see people filling up their bathtubs and just hanging out in there until things blow over.
More worrying is what happens to wildlife. Some animals simply can't escape like humans and most animals can. For example in Australia a recent heatwave killed something like half of all flying foxes!