There's no easy way for me to say this, but Dr. Tam was not being truthful.
In the week or 10 days.... Pardon me, I don't know the exact time span between her statement that masks were not to be recommended to the public and then changing her view to give permissive guidance that masks of a non-medical sort could be used. In that short period of, as I say, about a week or 10 days, there was no new evidence that emerged to justify that change.
There have been additional studies of masks, of course, some of it biophysics, what particle size will penetrate a mask in what conditions. But there was definitely not in that crucial window a game-changing study.