I will change my DS3's pooey nappy on the bus if he poos on the bus, but he has incredibly weak skin, and incredibly acidic poo as he lives off fruit veg and sod all else, and if a poo is not changed in under 5 minutes, his bum which could have been free of nappy rash previously, will actually be so bad it will be bleeding.
DS2 was the same, yet DD and DS1 could easily have waited till they got home.
Some DC's have rhino hide bottoms, others have tissue paper bottoms. Those with the rhino bum DC can wait to change them, those with the tissue paper bum DC have no option, in fact it would be neglectful to LEAVE them in that dirty nappy.
I think a bit of a smell (which you would smell anyway even if the nappy wasn't changed) is a lot less of a problem than a child's bottom BLEEDING.
OP, YANBU, I would have changed the baby. You know your own DC, and on a journey that long, it might be unavoidable.