I disagree. To me, 100% means 100% of that game.
To extend your reasoning to it's logical conclusion, your total gamerscore should be a percentage of the total available gamerscore out there - regardless of whether or not you've ever bought or played the game (so everyone would be at about 1% forever)
His gripe is that his percentage is penalizing him for not playing game content he didn't purchase. It's a valid complaint, in my opinion. Not a huge issue, of course - but still a valid concern.