[mindslight]

The problem is the difference between stating:

"masks have not been shown to be effective for the general population"

and something like:

"we do not know if masks are effective"

What was said was readily misinterpreted as "masks have ... been shown to [not] be effective for the general population", especially by a science-illiterate population reading with a non-individualist perspective. Whereas the second directly acknowledges the lack of understanding in an evolving situation.