When is it Okay? It's always Okay. But if I was one of the 5, my question to the airline would be, "When is it Okay to come by and collect my cheque for compensation?"
In this case it's past the original scheduled time but earlier than the latest estimated departure time. I take it this means that when the passengers checked in they were told the departure time was pushed back and were given a new estimated departure time. So based on what the airline told them, they are proceeding according to this new estimated time. And you'd like to know if its Okay to leave earlier?
A party to a contract can always be in breach, provided he knows that there is a penalty for breach which would be to have to pay damages to the innocent party.
So if it suits the airline to leave, then they can. I was at pains to set this out in another thread where USAir left 2 passengers behind so the others already on board said to be 122 could leave on time. That's surely a good decision and "Okay". Just compensate the ones you left behind.
And if as I have been hearing over the past day or two in the same thread that Americans are pretty self sufficient and its just the wimps and whiners who will complain, the airline has it really good. Of the 5, maybe 4 are self-sufficient and the airline only has to compensate the one wimp/whiner.
The airline just has to put up a sign
"Wimps and whiners this way to collect your travel voucher for $500 and a booking on the next flight out.
Self Sufficient Americans join the next line to get a new reservation bearing in mind that dates and times are not part of the contract"
See how quickly the other 4 self sufficient ones join the Wimps/Whiners line.