That’s not strictly true, because (historians believe) the Spanish flu triggered a cytokine storm in the bodies of the young and healthy, which would have dramatically raised it’s mortality rate and would have the same effect if it were the present day.
It’s also not as if we don’t have ridiculously fatal diseases today. H5N1 is a good modern-day example; H5N1 is a modern swine flu with a mortality rate of 60%, but it has only killed a few hundred since it emerged because the CDC hopped on that like you would not believe. Coronavirus’s low mortality rate is paradoxically what allowed it to become such a problem. A quirk of the modern age.