I don't think it's even that high. It looks like most GPUs are well under 1GH/s. If we assume 1GH/s, then at the current difficulty and exchange rate, if a page load gives you one minute of computation on average (wild guess, but seems reasonable), you're looking at about $5e-7 dollars per pageload. That's 5 cents/day if you get 100,000 people per day.

But this isn't exactly an argument that it can't be happening. They might have failed to understand just how futile it would be.