The union says there's no strike, the airline says there's no strike, no other airlines experienced similar failures despite similar mandates, and there's an alternative explanation that fits all of the available the evidence (and there's a lot of public evidence since these are domestic commercial flights). Additionally, people who want to stir up FUD about vaccine mandates have an incentive to lie.

So, one possibility is that there's a fast conspiracy that involves NOAA, the FAA, Miami airport admin, air traffic controllers, Southwest, the pilot's union, etc.

The other possibility a few conservative commentators are talking out of their ass.

Sorry... this one just isn't that hard.