From the article:
"While e-books are doing just fine, the fate of the devices dedicated exclusively to those e-books is much murkier."
How about it being simply due to a difference in useful lifetimes of devices?
My first e-reader, a PRS-505 is still fine. Along with Calibre, I can't see any limitations for reading on it just as well as on the first day I got it.
My first tablet, a Creative ZiiO bought about three years later, is running Gingerbread, so there are a lot of apps that have passed it by. It only has 512 MB of RAM. I've relegated it to video playback using their in-built ZiiO video app.
The used market has more usable older e-readers. People still like Kindle Keyboards from 7 years ago. A phone or tablet from 7 years ago is hardly worth the listing fees charged to sell it.
Longevity of the device's life, plus a deeper used market competing with new sales, seems an easy explanation for the author's conundrum. Then there's the anticipation of new tech, viz., a color e-reader, that can further put off device upgrade sales.