How is it impossible to measure? It has nothing to do with how his content quality ranks compared to other people. Since he is writing both the content under his name and the content under a different name, given that he doesn't know which one he's writing under until after he writes it, the quality should be at the same level (on average).

If the quality is the same but the points are different (between his real acct and anon acct), then you've found a bias.