If your point is that people were on the plane with them so they are now at risk, then that's a stretch not currently backed by facts. Generally respiratory viruses are most communicable when symptoms are initially presenting. These subjects were still in the incubation period and thus currently believed not to be in a communicative phase. Also, thus far it seems this particular coronavirus needs fairly close physical contact to spread, so the likelihood is pretty high that anyone on those planes who goes onto to develop symptoms was already a carrier themselves.
The concern is that because this seems to be a newish virus (in terms of making the jump to humans), it could quickly adapt to incubate more rapidly, and spread more easily. That's what has China spooked.