Aye, that brings up another issue. What exactly does “airborne” mean? The assumption early on was that Covid was not airborne, and required droplets of a certain size, though I think that changed over time.
We might never know this about COVID as further studying it can only uncover more malfeasance at this point, but traditional airborne stuff like smallpox or measles won't be affected by a dirty bandana on one's mouth or, even, over the nose.