As somebody just flying the line and not heavily involved at the time, this "certainty" of which you speak was anything but. My perception was how Fish described it.

United had just rolled out their vaccine mandate, to which ALPA responded with silence/concurrence (I can't remember which). Shortly thereafter, the Southwest CEO announced its mandate. The next few days Southwest canceled thousands of flights due to "ATC" and "weather." No other airline, or air line, was similarly affected. Within a week, Southwest's CEO rescinded the mandate.

It was only after this failed SWA mandate that DAL announced its voluntary vaccine policy. Coincidence? Maybe. However, if a voluntary informed consent policy was always the case at Delta, they did a fabulous job of withholding that information.

A5S

Edit: I believe there are UAL pilots that were fired for not taking the vax that still are not back on property.