It's a tricky subject for sure, since the res & color restriction alone wouldn't disqualify a photo taken with a gameboy camera, for instance.
It is the stylistic intent and process of arriving at the result which distinguish between low resolution art and pixel art, no matter how low it goes.
It would be pretty incredible if someone were able to produce a polished bit of pixel art at above gameboy color res in an hour though, so that makes sense to me for xhbs. For majors it could to be higher definitely, like maybe mega drive territory or even VGA.
Personally, I think 640x480 is a bit too low for draw, if the goal is to distinguish it from pixel— generally, 640x480 is still definitely viable for detailed pixel art, though painstaking. My demo.mid sub for last future battles was 600x600. Maybe 800x600+?