Yes 'H' is for the horizontally-opposed layout

  > H = Horizontal ?

  Good thought, though I doubt it. I think Subaru's just trying to be
  clever. What's wrong with "boxer"?

  Orientation of the engine shouldn't have anything to do with
  engine-layout; an inline engine runs along the axis of the vehicle; a
  transverse engine lies across it; a flat engine has horizontal
  cylinders (but could be a straight or so-called "inline" engine).