But name a time since the dawn of commercial air travel 1950-2019 where a country, much less the country with the largest population on earth, suspended all air travel for any reason, much less a flu they'd only discovered a few weeks prior.

I have my doubts on how many other countries would've isolated their entire population and suspended travel in and out if given the same time line and information.

Do you really think if a flu outbreak had been detected in the US that we would have suspended ALL travel in and out of the country in December of 2019 only a few weeks after becoming aware of it?

It's been a century since this last happened, no one alive in 2019 really had any memory of such a catastrophe unfolding. I think China would've been called alarmist, totalitarian, and an unreliable business partner, had they taken the full measures of lockdown early enough to possibly prevent it spreading internationally.