There was a time when any FPS game was referred to as a Doom-a-like or Doom clone. Heck, eventually even tihngs with a first person view but no shooting, or that pre-dated Doom (Doom was a step change in a few ways but it certainly wasn't the first FPS, I remember some more primative attempts as far back as the mid-to-late 80s). The current referring-to-everything-as-a-doom-clone is basically an extension of that.