They do exactly that "which is the better answer?" test -- I've seen it pop up a few times.
"Which diamonds are shinier, the blood diamond sourced ones or the ethically sourced ones?" ... that's not the same question as "which diamonds are blood diamonds" (to employ an extreme analogy)
Concluding that no one could detect which ones were blood diamonds because they were "equally shiny" is not really correct now, is it?
And the Daring Fireball article does complain that watermarking will reduce quality. If that's what you're trying to check, "which is better?" is the right question.