well, no, it wasn't, but an empty seat next to them meant there was an empty seat next to them, not that there were other empty seats, it seems that the couple asked if they could change seats, which would seem to indicate there were other empty seats, but the couple may have just wanted to have other people take their seats instead of them
I did a bit of digging... Apparently airlines don't want to officially pronounce someone dead until they're off the plane for liability sake.
If the family wants to sue, they can request an autopsy and determine the time/cause of death, of course. But airlines waiting until they're off the plane adds another layer of friction in the process of pressing charges.
How awful for the families, in an already horrible situation.