If there's an epidemic starting, then only a small number of people have the disease. Everyone wearing a mask would help, but if you have a limited number of masks, you want to focus them on people likely to have/catch the disease.
The masks worn by people without the disease would be ineffective or even counterproductive if health care professionals were missing PPE as a result.
> if health care professionals were missing PPE as a result
True, but "don't buy masks NOW because the health workers need them" is totally different than "don't buy masks because they are not effective for the general public and can actually be worse" - being either a terribly wrong idea (like the article defends) or outright lying to the public (if the message was told not because it was believed to be true, but as a roundabout way of being extra emphatic to avoid the public hoarding masks and causing PPE shortages for health workers).
The masks worn by people without the disease would be ineffective or even counterproductive if health care professionals were missing PPE as a result.