You're right, there's no proper way to identify star players. I'd read that somewhere though - must have been a misinformed statement. What you say does make more sense - average or total salary is a more definite measure. I just thought it was funny that you can get more tv revenue because you have more players sitting on the sidelines.
i don't know how it is realised, but maybe the system determine the impact of the player by the salary structure in the team.
So that the best paid player, have the most impact on tv money while number 15 didn't get considered. Maybe this would be a way to consider the "stars"