If masks turn out to be ineffective, the cost of having them recommended is reduced reputation and authority.
Would you take security advice from Bruce Schneier after he proposed you to cover your keyboard in snakeoil because it might protect against computer viruses (and if not, there is not much damage)?
Regardless of how well they work in practice, there's a clear theoretical reason why masks could/should work: they catch salivary droplets containing the virus.
Covering your keyboard in oil is not even theoretically going to affect your computer's susceptibility to computer viruses.
That’s the difference between theory and practice.
In practice the snakeoil may prevent you from using your computer more frequently and therefore result in less infections.
In practice I observe a lot of people fumbling out unwashed pieces of cloth from their pockets and placing them in their face, hoping it would preserve their respiratory health.
Would you take security advice from Bruce Schneier after he proposed you to cover your keyboard in snakeoil because it might protect against computer viruses (and if not, there is not much damage)?