You'd think there'd be enough competition for carrion in the tropics, not only from other insects like flies feeding their young by laying eggs in it but many other forms of life, to make nectar appear far less competed for.
It's also 180° opposite of honey, an antimicrobial substance, as food for the bee larvae. And on top of that to further putefry the harvested meat inside the hive for 2 weeks in a tropical environment? Is that their equivalent of sous vide, or low & slow minus the smoke, or????
I suspect there's a lot more in play here than competition for nectar, that goes back a long way in the evolutionary history of these "vulture bees". Just because this wasn't necessarily observed previously by scientists doesn't mean it hasn't been happening before any scientists observed or tested this behavior.