In the past few days it's been determined that this was a lie on the part of the photographer.
José Carlos Meirelles has admitted that authorities have known about the tribe since at least 1910! He took the photos and still claimed that these people had been completely undiscovered until his aerial reconnaissance. Why? Because he wanted it to seem like they were a truly lost tribe so that more attention would be given toward the issue of the impact of logging in the Amazon basin.
In other words: it was a lie driven by a political agenda.
I still harbor some admiration for the photos that Meirelles took, but now any appreciation for them will be forever tainted by how he chose to use them. If he had just come forward and said "hey, these are pictures of a tribe that we've known about for a long time but only now are able to get close enough to photograph them" that would have no doubt been a more respectable feat. He didn't play the part of the objective scientist at all. Instead he injected a personal bias into the matter and in the long run he probably did more harm than good to his cause.
That's a lesson that many other scientists would do well to be mindful of.
(And thanks to Nathan for passing along the news about this.)