> The officials, speaking on the condition of anonymity, said that about 400 volunteers participated in the serology tests, lower than the 1,000 volunteers that were sought, but enough to provide statistically relevant data about how the virus spread aboard one of world’s largest warships.

There could be a correlation between the probability of getting tested and the probability of having had COVID-19-like symptoms or known exposure. In this sense, the Navy study resembles the ones that sampled from willing shoppers in front of grocery stores. Drawing conclusions isn't very easy here.