90%? Considering the near-ubiquity of the handheld and its expanded casual audience, we highly doubt that even 9% of DS owners in the US have a flashcart of any sort, let alone know of their existence. Though we don't consider ourselves supporters of piracy, stating a false figure like that as fact just to rally people to his cause is unconscionable.
In a way, we kind of wish the R4 was that popular, but not for any illegal reasons -- it would likely result in more homebrew projects and a much larger homebrew audience. Mr. Hillier will hear none of that, though: "The R4 has shifted balance of power in the piracy industry to the consumer -- and that is hugely worrying. That's why we intend to stop trade in these chips wherever we can."