He has a point, I realized after a moment's thought. To me, Alternate History is science fiction because everyone knows AH is SF. I've written a fair number of AH shorts ("The Cleansing Fire of God," "The Righteous Path", "Our Lady of American Sorrows"). I've always thought of them as SF, but I've never thought carefully about why they're SF.
In other words, I don't have a more rigorous realization than "because." Why couldn't AH simply be it's own genre? Is it inherently science fictional?