The incubation period runs out to around 27 days, though above 2 weeks is uncommon. It seems possible one of them already had it before quarantine, and just happened to be one of those who had a long incubation period.
I obviously don’t know whether that’s the case here but its also possible that some of these anecdotal stories (I’ve heard similar from colleagues) involve someone in the same household having had an asymptomatic case acquired prior to quarantining and transmitting it to others in the same household who then get symptoms after a week or two.