No, they didn't just say that, that was the problem. They said that there was no evidence to support the idea that masks were effective against COVID-19. This is the usual weasel word approach of saying "there is no evidence" to support something to lead people to thinking there is counterevidence, which there wasn't. Once they started lying to people this way, it was over.
The proper messaging was: "we need to keep the mask supply up for medical workers if it turns out masks are effective in helping reduce their exposure. we don't know yet if masks are effective, they might be, and we are researching this right now as fast as we can. in the meantime, it seems prudent to assume they may help, and we ask the public to avoid buying masks for the time being while we increase the supply so our medical workers can have them."
The proper messaging was: "we need to keep the mask supply up for medical workers if it turns out masks are effective in helping reduce their exposure. we don't know yet if masks are effective, they might be, and we are researching this right now as fast as we can. in the meantime, it seems prudent to assume they may help, and we ask the public to avoid buying masks for the time being while we increase the supply so our medical workers can have them."