What I've read about Coronaviruses is that most transmission is either through hand to face contact or droplet-suspended virions so a mask that doesn't protect against aerosolized droplets would still be fairly effective. I understand that surgical masks aren't intended to prevent inhalation of droplets but the substantial symmetry between air passing through one way or the other would suggest at least broadly similar effects either way. Given the empirical results from elsewhere in the thread it seems like that may be the case.