Except that the bubble was broken when they boarded the new passengers.
I wonder if they just didn't properly understand the concept of an incubation period that could be upto 14d long and the implications of that. And then add to that the possibility of false negative [even multiple times] tests.
Or perhaps they did, and decided the risk was low and it was "worth it" to give a "normal" cruise experience.
Either way, it appears it didn't work out so well...