"F/As are not paid until the door closes" is generally true, but it is an oversimplification. It is true for a normal month and a normal schedule, but there are times when it is not.

Pilots and F/As (crewmembers) will also have a minimum monthly guarantee that they are paid when they pay credit for the month is lower than guarantee. That normally doesn't happen, but there are times where it does.

Crewmembers can also be paid a trip rig, duty rig, or minimum day rig when the trip that they're flying is relatively inefficient and the actual block times of the trip are less than the rig time.

The pandemic payroll support programs prevented participating employers from involuntary furloughs (layoffs) while they were receiving the government aid. That resulted in the airlines having an excess of crewmembers over their need to cover the reduced schedules. That resulted in a lot of crewmembers receiving their monthly guarantee as their actual flight time was well below guarantee. In those cases, the time that the door closed was irrelevant to their pay.

Without the payroll support programs, the ramp up would have been restrained, even more than it has been, by a lack of qualified crews. Even more so for pilots due to the long training cycles and limited training capacity.