> If it's already out, how will stopping flights and closing schools really help? How long will you stop them? At best, this will just slow the spread, not stop it.

Yes, it will.

One thing that tends to get neglected is that in a lot of diseases, most people don't pass the disease on very well (maybe infect one other person) but a small number of people (superspreaders or superinfectors) pass it on EXTREMELY effectively.

If you can block the superinfectors by quarantines, that is like putting control rods into a nuclear reactor--you slow the progress dramatically.