"He said it's likely passengers brought the virus with them when they boarded"
They always say the passenger brought it on. Absolves them of some blame and the extra costs regular decontaminations of onboard drinking water systems.
Well, it's entirely possible.
A highly contagious thing gets on a closed system of closely packed people, and it can spread like wildfire. Serious flus and plagues have spread like wildfire in very open areas.
In any case, it is not encouraging. Makes you wonder what disease-ridden person you're getting on board with who'll knock everyone flat.