There's a short story titled "Ancient History" that (with a heavy dose of inference/speculation) seems to suggest the Men of Stone were humans engineered specifically to have no/a greatly reduced warp presence in order to avoid attracting demons while navigating the warp.

And (with an even heftier amount of speculation) the attributes seemingly ascribed to the Men of Stone line up suspiciously well with what's known of the Leagues of Votann.