That's exactly what I wrote: the working assumption was that you wouldn't be contagious (because then you'd be staying at home yourself). That turned out to be wrong. Later.
Given that this assumption doesn't work with something as banal and common place as the flu... why would it apply to a new respiratory virus, especially since people were anyways coming down with colds and the flu and had no clue what they had.
To be fair, I remember looking it up on the CDC website when this all started, and they explicitly stated any masks are not necessary (or useful) unless you are caring for a sick patient.