I think the comment is positing that each major city making it's own preparations is inefficient and ineffective. What if the pathogen escapes that city and is now floating around the country?
We got lucky. Covid19, while deadly, is not as deadly as it could be. If we'd run into Spanish Flu, we'd be in a world of hurt right now. The more deadly pathogens out there are just not to be met with half measures. If you're talking Spanish Flu, you don't want to be putting Dallas on lockdown, and still have who knows how many people on the road to Norman, or Tyler, or Nacogdoches. All because they left before the city of Dallas was able to close the city. The city of Dallas would not even have the ability to know where those people are, or where they are headed, or what planes they are on, or what cars they are in. There really are no other options here, if you want to ensure maximum survival, you must have the coordination be at the federal level.
Again, with something like covid19, it probably doesn't make much difference. Sloppiness is not going to kill 10 or 20 percent of your population in the case of covid19. So we can tolerate the kind of Keystone Kops thing we've got going on right now. But if something comes along like Spanish Flu? I'm sorry, but we shouldn't be pussy footing around with a bug like that.