“The Spanish Flu was not mitigated on any significant scale”
That can be disputed. There were plenty of quarantines, calls for social distancing, hand washing, etc. Time to travel was a form of mitigation inherent in the technological level of that time compared to now.
But the differences I see as most significant are the spread of the disease by asymptomatics, the contagiousness of it and the mobility of the population.
There were not tens of thousands of college students flying home to everywhere in the country from Spring Break in 1918. For example. Catch it on the beach, jump on a plane, give it to Grandma three days later before you even have symptoms or even the person you got it from has symptoms. Even if you both end up on ventilators later on.
The reason this became the monster it did was because it came right smack dab in the middle of the Chinese New Year, which is the largest human migration on the planet, with millions of Chinese traveling across the globe to be with family.
There may not have been tens of thousands of students flying home, but there were millions of soldiers transported around to fight WWI.
But we did have a million young men under arms in very close quarters.
But the SF came on so fast, most of them did not have the ability to go very far before they were locked down.