I think that the only thing that would make someone who only jumps tandem "not a skydiver" would be if they do not take the initiative to learn about skydiving. For example, if someone does tandem after tandem and just goes WHEEEEEEE! .. taking no interest in learning about the gear, canopy control, winds, landings, etc, etc, then they would NOT be a skydiver. There is so much more to our sport.