Flight crew regularly do depart with less crew than they're entitled to: witness downroute disruption or any delayed 3 man HKG or 2 man MIA. They're entitled to go back to stand for the extra man but they don't.
In the case in question only the magnanimity of the Captain kept Vicky Pollard on board. With an extra standby called out it's clear that the he'd already considered offloading the trouble maker and could have done so should he have wished. I suspect it was only the presence of his/her case in the hold that kept the insubordinate one on the trip. Don't make the mistake of thinking your union is in charge if the aircraft. It isn't, and if you give the commander lip you'll be waving goodbye to the aircraft from T5.