There's a weak link somewhere--I wonder if it's flight crews in this case. If the pilot and other crew for the leg from South Africa to Antarctica weren't also isolating 10 days and tested at the same time, then there's your infection vector.
There’s also the factor that some people have a longer incubation period. Early on in the pandemic there were reports of some people having 21-27 day incubation periods before testing positive and getting sick. I think it’s less common now with delta and omicron but likely still possible in rare instances.
