The hivemind is an analogy for the way the average person had turned on the war and viewed it as a national disgrace. Just watch that opening act of Rambo, the soldiers returning from Nam late in the conflict came home to joblessness and derision. A far cry from the parades and support of the GI bill the generation before had.
Haldeman is trying to show a universal truth of long wars, at some point the home you left stops existing, and you return to a place that isn’t what you fought for in the first place, though of course Haldeman takes this to absurdity almost. Like many vets of Vietnam Mandela and his fellow soldiers are now so feared and removed that society doesn’t have a mechanism for integration. Haldeman uses genetic and cultural shifts to represent the more subtle social ones.
My parents started calling me it about thirty years ago; it’s either an allusion to the fact I have broken so many bones, or a tacit admission that I was an unplanned pregnancy. Or both.