I'd argue it has always been known that masks would protect both patients and wearers even from first principles and also how past pandemics with a less infectious virus were handled in Asia and from how common cold and flu dropped as a side effect.

The virus spreads via droplets exhaled or emitted from nose and mouth. What happens when something obstructs that flow - the % of droplets going in & out & velocity decreases and likelihood of transmission reduces. Why do you think surgeons wear masks - it's both to protect the patient and the surgeon from droplet borne infections.

What was unknown was whether it was economical to wear masks widely and whether there were supplies, not that it would work.