Not true. Integrating elephants, even cows, is fraught with danger.
Even the sanctuaries, with their vast areas of land, have found that they need to keep many cows as singletons because of their incompatability with others. Unfortunately we only get shown pictures of the compatible animals roaming "free". The singletons, in much smaller accomodation, aren't good for propaganda and so we rarely get to see that side of the picture.
I'll bow to your superior knowledge on this one, the people I know handled Indian elephants brought to Africa for training African elephants. And they had few problems, maybe they were lucky.
I have personal experience though of wild elephant cows joining other herds and being accepted, not always, sometimes they get drivien off, but in times of short food supply even herd members can be driven away.
But Auckland Zoo were planning on getting another elephant or more anyway, so the problems of incompatibility with another elephant didn't bother them.
The main reason they gave was that the different species would not mix, and African elephants carried deiseases which might prove a risk to their Indian. Which I know to be wrong, both elephants have lived in NZ most of their lives, and would no longer have any diseases from their home countries. And there ar eplenty fo zoos where African and Indian do live together.
It's also been said that tigers and lions can't be put together because they will fight, or adult lions can't be put together for the same reason.
Not always the case, there are plenty of examples where adult lions and tigers have mixed sucessfully.
So they could have at least tried to see if they would like each other, Auckland zoo is set up to seperate elephants if need be.