Based purely on anecdotal data taken from one single user - moi - I would say that those numbers are at least spot on, perhaps even a tad understated. Pre-eBooks I made something like three or four purchases a year from Amazon, around six or seven books at a time. That would total around 40 books a year. Including S&H I probably spent $500 per annum.
During 2011 I downloaded and paid for 147 digital titles and spent $738.68. And noo, I haven't read all of those yet, but they are all on the Kindle and catalogued in Calibre.
So yes, I think there might be some validity to those numbers.