So first, i don't think that article says they tracked them all.
And second, if you can test them and rule out infection with certainty, why would they be ordered to isolate?
I think the incubation period does play a role there.
Authorities in St. Helena, the volcanic British territory in the South Atlantic where passengers disembarked, said they were monitoring a small number of people who were considered "higher risk contacts." Those contacts were being told to isolate for 45 days, the St. Helena government said.