There is a very good explanation from MedCram how this novel coronavirus can spread https://www.youtube.com/watch?v=nW3xqcGidpQ
There is specifically about this event (at 5:47) how Germans and now how it turns out Spanish people contracted the virus.
What is interesting is that people can be shedding the virus not only before they develop symptoms, but also after they recover from illness. Perhaps it is only those who undergo mild symptoms, however, in such case there are lots of possibilities for the virus to spread.
I wonder if current testing can detect the virus in those who don't have symptoms but have the virus and are contagious. It seems that it is not possible now, otherwise why would all those evacuated people would be held quarantined if they could just be tested for safety purposes.