The convention defines “secret history” as tales which uncover an alternative history of our world with the aid of fantasy literary devices. Like alternate histories or secret tales of the occult.
A secret history might also mean a lost history, something written in a language that died with the last native speaker. It might mean something inaccessible, written in a medium too fragile to last. Like the science fiction and fantasy stories published in U.S. newspapers in the late 1800s. We know a few of those authors, like Aurelia Hadley Mohl and Mollie Moore Davis, but how many others were there? Those stories were proof that everybody has always been here, but the paper they were printed on has turned to dust.
We might know that C.L. Moore wrote for Weird Tales, but I grew up thinking she was the only one, that a woman fantasy writer from that time period was like a unicorn, there could only be one, and that she was writing for an entirely male audience. But there were plenty of other women, around a hundred in Weird Tales alone, and many of them, like Allison V. Harding and Mary Elizabeth Counselman, didn’t bother to conceal their identity with initials.