It all depends on the companies and institutions. If there are no special holidays or orders, people will go to work no matter what and the epidemic will spread. On the other hand, if the government or companies imposes a quarantine, it will be respected.

The problem is that as far as we know about 15% of the people who get infected with covid-19 require intensive care. Combine that with the fact that covid-19 is very easily transmissible (even from people who show no symptoms) plus the size of a megalopolis such as Tokyo and things could get really ugly really, really fast. No government has tens of thousands or even hundreds of thousands of hospital beds at the ready in order to handle such a scenario (and this is also why the Chinese had to resort to putting sick people in sports halls and the like).

There’s also the very ugly fact that most probably half of the health workers directly involved will also get infected in the first 3 or 4 weeks, at which point you’d need to have a fresh batch of new doctors and nurses easily available from the outside.

There’s (probably) also way more people together. I doubt having a clean country helps if the virus is transmitted person to person.

I’m not saying the same number of people will die (because better and more prepared healthcare), but it’s a fantastic place for another outbreak.