This whole "authenticity" thing is kind of tricky.
Imagine some object -- such as a Chippendale chair or a Ming vase. Now imagine it is possible to make a reproduction of that object, so exact in materials and workmanship that no expert, nor any testing method can tell the difference. Most people would be delighted to own the original, but horrified to learn they have a reproduction, even though the two are indistinguishable. Lack of authenticity violates our sense of fairness even when there is no logical reason why it should.
If I photograph a Scotsman, let's say he is a Campbell, and I provide a plaid for him to wear, and that plaid is one of the tartans favored by the clan MacDonald.
A typically ill-informed viewer, myself included, might admire the portrait, and say, "That is a very fine portrait of a Scotsman; I really like that!" But someone who is familiar with the history of the Scottish clans and their tartans, is going to take one look and say, "WTF!" (or whatever is the Gaelic equivalent)
The success of a photograph depends on what we can see -- subject, light and composition. But how much does it also depend on what we know? From a strictly aesthetic point-of-view, putting a Campbell in a MacDonald tartan is not going to make much difference (especially so in black and white). But when we cross from the photographer's side of the asile to the ethnographer's side, putting a Campbell in a MacDonald tartan would be unacceptable, suggesting ignorance, laziness, or dishonesty.
But as
@koraks has said, it's hardly fair to judge Curtis by the modern standards of a field of study that barely existed in his time. I am satisfied to enjoy his photos for their esthetic qualities and for the way he shows us the nobility of his subjects. Still, knowing what we know today, I would be uncomfortable if a modern showing of his work failed to mention, if only as a footnote, that his work may not always be an entirely authentic representation of his subject.