Only it isn't a myth.

"The typical incubation period for COVID‑19 is five or six days, but it can range from one to fourteen days with approximately ten percent of cases taking longer."

https://en.wikipedia.org/wiki/Coronavirus_disease_2019

I could have been more correct by writing 'up to', but if 10% of the cases take longer than 14 days then 99.5% within 14 days should be 90%.

Playing it safe with the incubation time around international arrivals is what has led to authorities the world over adopting the 14 day quarantine.