It may have come down to when they pressed the “use the spare” button. And it would certainly not be 1minute past the scheduled departure. I recall one of the aviators here saying there are different levels of being on call in terms of how close one needs to be to the aircraft door. Certainly, it would not be like the military - where the pilots are all dressed and scrambled within seconds.
Speculating:
Let’s say they pressed the button 1 hr after sched departure. Then allow say 1 hr for standby crew to arrive. Then get the standby aircraft to the stand 1hr. Then crew briefing, aircraft loading including pax, fuelling say 2hrs. That would be easily 5hrs.
