In a way I wish more people did this, ideally before boarding, and the airlines made it easier to just be rebooked on the next service. Admittedly easier said than done given how full BA's flights are at the moment. But otherwise it would be a better world if people having arguments could just pause, have a coffee, reassess things, talk a bit and listen a lot. Fairly basic stuff and there is actually some science behind this, but it probably needs encouragement. Once people have boarded, at a certain point if it gets too difficult to reconcile baggage then it may involve offloading everyone and starting boarding from scratch, so while 4A had an unfortunate outcome, it could have been worse. Hopefully their bag was sent on fairly soon thereafter.

The text would have come from BA automatically once the delayed bag had been marked as such in the computer, probably down in LHR warehouse. The status change triggers this message. One reason for checking your mobile number is properly recorded in the booking.