The mask push then was because, for a while (and despite evidence of infectious agent passing through too-small-for-a-bacterium filters) they believed influenza was being caused by a bacterium, not a virus. Even then, they knew that the masks available would not stop airborne viruses. And masking didn't work then either (areas with enforced masking fared no better than areas where masking was optional). You're welcome to dig into accounts of the pandemic and medical journal articles written during and in the immediate aftermath of the influenza pandemic and confirm this for yourself.

Likewise, the initial (honest) advice not to bother with masks for covid was based on the fact that covid, like other coronaviruses and influenza, is aerosol-borne and aerosol particles pass through surgical mask material easily (so wearing a 2nd tight-fitting mask over a surgical mask to get a better fit doesn't matter).

The u-turn on masks for covid wasn't based on science. The justification later given was that the earlier, correct advice not to bother with masks was a noble lie to conserve supplies of masks for healthcare workers. At the time, the reversal was publicly predicated on hand-wavey (and false) claims that covid might be droplet-spread (droplets are larger than aerosol particles and droplet emissions could conceivably be greatly reduced by well-fitted surgical masks with no gaps).