I did some research: Amazons actually existed in
Tolkien's
First Age in the House of Haleth. But firstly, they were not a tribe of their own, they were rather the bodyguards of their female ruler at a certain point in time. Secondly, of all the
peoples of
the Third Age, the House of Haleth was most closely related to the
Dunlendings, but not to the
Rohirrim, who were the enemies of the Dunlendings.
The Rohirrim also had shieldmaidens, but they were not a separate ethnic group, let alone a lost one.
Conclusion: There were definitely isolated female fighters in
Middle-earth, but there was never a large group of lost, rejected or otherwise emigrated female fighters.
If there had been such a group, and Hera had led them to save the Rohirrim, then Tolkien would of course have written about it - unless he was a chauvinist. Otherwise, unfortunately, this fanfiction makes no sense.
But I would still remain optimistic for the time being and assume that the article is fake news, otherwise it would be very stupid of the producers to upset the fan base so much.
One thing is certain, however: there is no “long-lost tribe of female warriors” in Tolkien's
books with regard to the Rohirrim.
It also makes sense, as there was never a tribe of female warriors in real history. As I said, this only exists in Greek mythology, and this is probably invented. At the same time, it also makes sense that there were always isolated female warriors when the need arose. However, to ensure the survival of a people, in general it was always necessary and sensible for women not to go to war, with a few exceptions.
Either way, it makes no logical sense for women who are gifted in war to secede en masse from their native people and then live somewhere in hiding. This is either an ancient fantasy or modern feminist nonsense. But in no way compatible with Tolkien.