In a striking real-world experiment, flu patients spent days indoors with healthy volunteers, but the virus never spread. Researchers found that limited coughing and well-mixed indoor air kept virus levels low, even with close contact. Age may have helped too, since middle-aged adults are less likely to catch the flu than younger people. The results highlight ventilation, air movement, and masks as key defenses against infection.

TL;DR - factors that could have potentially resulted in this is - less coughing by infected patients, good ventilation, volunteer adults which were healthy were mostly middle aged which is less likely to be infected. We need to do more trials to understand how seasonal flus work.