I believe the premise behind it is that COVID-19 is carried via droplets that are created when you breathe/talk/cough. The masks are supposed to reduce the amount and distance these droplets can travel. While masks are considered a medical device, the guidance provided by the CDC, etc. likely circumvents the case of someone practicing medicine without a license. I'm thinking the court is going to view it the same as a company requiring an employee to wear a hair net, long sleeves, etc.

FWIW, I'm not a lawyer.

By your logic, wouldn't it be "medical advice" for an employer to force an employee to wear gloves when touching food because wearing gloves supposedly reduces the transmission of diseases like hepatitis? There's research out there suggesting that bare hands can be more sanitary in a lot of contexts, so it's not like it's proven that gloves are always better - it is just a legal requirement. Or, wouldn't it be medical advice for an employer to force their staff to serve food with tongs, even though they aren't legally required to do so, because it's supposedly more sanitary (even though there's research suggesting the opposite - cross contamination risks, etc.). Obviously not - you don't have to be a doctor to try to make your workplace more sanitary, even if your efforts are misguided.

As long as you're not literally pretending to be a doctor/nurse/licensed professional, enforcing broad beliefs or even providing advice about how to prevent or treat illness in general is not "providing medical advice". Beyond pretending to be a doctor to seem more authoritative, illegal medical advice is stuff like actively diagnosing or attempting to treat a specific person's specific illness - stuff literally only doctors can do.