Wouldn't that only be true for fantasies that were set on Earth? I think that Speculative History can be a subset of Fantasy or Science Fiction, depending on where you take it. Imagining a magical Roman Empire is one thing, imagining a world where the Axis won WWII is another.
Terry Brooks' Shannara series is basically set after an "
Old World"
was destroyed through nuclear war...so is it Fantasy because it has magic and monsters or Science Fiction because it is in the "
future"
? I think most people would say Fantasy. And it's not historical because it's set in the future...there were no decisions in the past that changed. Robert Jordan's Wheel of Time suggests that history is circular, that everything happens over and over again with small changes each time, and the story in his books could be our future or our past, we have no way of knowing. It doesn't require any changes to our own history to work.
I'm not saying that you are wrong, really, just that you made a pretty sweeping generalization that I'm not agreeing with.
Applicable in some cases, but not in many others.
Game on,
____________________________
"
He who is certain he knows the ending of things when he is only beginning them is either extremely wise or extremely foolish; no matter which is true, he is certainly an
unhappy man, for he has put a knife in the heart of wonder."
-- Tad Williams