To be fair, that's not a very good analogy. Now if you had a percentage of the population that enjoyed fantasy books, rather than were just literate in English, that might be a more fair comparison.
The court docs specify "hundreds of thousands" of "core books" and 6 million "D&D players". The number sold could be as low as 200,000 and still qualify as "hundreds of thousands", and "core books" probably encompass at least the DMG, MM & PHB (and with 4e it could arguably mean all books sold to date). Assume the PHB has the lions share of that (say 50%) and we could be looking at as little as 100000 PHBs sold. Given that the goal of 4e was to appeal to new players as much as existing fans, and given the total existing fanbase of 6 million... that's only 1/60 of the existing market that bought into 4e (assuming the goal of attracting new players failed - if it didn't that %age of market penetration is even lower).
Of course, having said all that, there are a lot of assumptions in play here. We could say between 1/6 and 1/60 of the existing market bought into 4e. Dunno about you, but that isn't exactly what I'd want to see as a sales/marketing exec. Maybe that's why WotC are flailing around after pirates...