If the fossils of a rhesus were older than the fossils of the chimp, and there were no human or chimp fossils as old as the rhesus fossils, why would that be an invalid conclusion?
I hope you won't think I'm dodging your question when I say that's exactly my point.
We know that we didn't evolve from chimps...but what if we didn't?
For all we know, Australopitheucs currently occupies the same role in real evolutionary theory that the chimp occupied in my fanciful scenario. Maybe they just say she's an ancestor because there must be one at about that spot.