Do you think it sounds too high or too low? It doesn't seem outrageous to me.
In the Portland region 72[1] people were reported to have died from it.
Going by the metro region with a population of 2.2 million, and assuming the heat-related deaths were underreported by half (round up to 100) that’s .0045%
[1] https://www.opb.org/article/2022/06/27/portland-remembers-pe...
[1] https://www.nytimes.com/interactive/2021/08/11/climate/death... - see graphic
Heck, it hardly makes news when Silicon Valley breaks 100F these days.
Another thing to consider is heat waves are much worse in regions without reliable or universal air conditioning. This could explain the much higher death tolls in South America compared to North America.
That is deaths per pop, not rate of deaths. Assuming people live to 80 on average, you'd expect 1/80 to die each year. So multiply that number by 80 to translate it to per death instead of per pop, and you get around 0.36% of deaths that year in Portland was due to that heatwave. 0.36% line up really well to the reported 0.4% figure from the article.