Oh indeed, it's just where do you draw the line? If a car has a very small range-extender engine that's only used occasionally, the occupants' exhalations could rival it!
Hybrid cars are already a thing — and a neat thing at that, I'm glad they exist — but I'm not the one who gets to draw the line, and the people who did, have done so with (hopefully) better forecasts than I have access to.
It's not generally considered an important factor.