The sentence following the one you quote says: "However, during a severe pandemic when use of face masks might be greater, pandemic transmission in households

The sentence following the one you quote says: "However, during a severe pandemic when use of face masks might be greater, pandemic transmission in households could be reduced." Later: "Although our study suggests that community use of face masks is unlikely to be an effective control policy for seasonal respiratory diseases, adherent mask users had a significant reduction in the risk for clinical infection. [...] Adherence with treatments and preventive measures is well known to vary depending on perception of risk and would be expected to increase during an influenza pandemic. [...] Therefore, although we found that distributing masks during seasonal winter influenza outbreaks is an ineffective control measure characterized by low adherence, results indicate the potential efficacy of masks in contexts where a larger adherence may be expected, such as during a severe influenza pandemic or other emerging infection."