Color me skeptical on this report.
The incubation period for the flu can range from one to four days, according to the CDC (at least according to the CDC on the internet).
“Symptoms of influenza can start quite suddenly one to two days after infection” -wikipedia
Is it possible that those who showed signs of the infection in such a short period of time had some sort of compromised immune system? I’m no doctor, and I don’t play one on TV, but I was always told that by the time you shows sings of having the cold or flu virus, you were probably infected 1 or 2 days previous