I can’t recall their exact messaging anymore, but I know at the time I got the impression that they said to save the limited supply of masks for the people that really need them. I remember constantly arguing on the internet at the time that that was what they said. They didn’t say that they don’t work, they just said not to start buying and hoarding them away from the people that need them.
But I guess one specific sentence caught “the other side’s” attention more than the rest of the message. Probably because they never actually check the source for what was actually said, and only read small clips from it that they have been told to react against.