My only gripe with the article is one of numbers - we retailers get more like 40-45% off on D&D books from distributors. I've also heard from distributors that their margins aren't anywhere near what ours are, so
@Alphastream, your later conclusion that WotC probably gets more than 25% jives with what I would expect. It's not Retail 60 Distributor 30 WotC 15. It's probably more like Retail 60 Distributor 35 WotC 25. With a big chunk of WotC's profit being lost to printing and shipping costs (shipping from the printers, the other shipping costs are passed on to us, much of the time).
But your conclusions look pretty close to right to me - nothing I could argue with. They're a guess, but they're as close to reality as I can imagine. I would guess (and this is based on nothing more than gut and the numbers that I've heard) but I expect that DDB sales are somewhere between a quarter and half of total print sales (and growing).