Accessibility has a specific meaning when applied to the visually impaired. I'm sure that you know that the Kindle (like many other devices) will read you book to you? You might think that that would mean the blind could use the Kindle. Well, no. You still can't really use the Kindle unless you can read the menus.
Accessibility means being able to do anything in an app without being able to see/hear what you're doing. We're talking about reading apps, so the hearing part isn't relevant to this discussion. It's the visual component that matters.
B&N have added spoken word cues to all the menus in NookStudy. I can't say how well it works, but I can say that this is the only major reading app or ereader that has this ability. Blio, for example, was pitched as an accessible reading app, but it never got beyond TTS. (Most of the major reading apps never got as far as TTS, so I suppose Blio did something, at least.)
When NookStudy launched last fall, it was one of the best academic reading apps on the market. Now it is moving into a second niche where it's going to challenge all the expensive accessibility apps. If B&N can keep this kind of innovation going, they are going to be a force to be reckoned with.
One last note: there's a mindset that successful tech companies have which many in the book industry lack; it's part of the reason why Amazon are winning. I'd say that B&N have adopted the tech company mindset. If any bookstore chain survives the digital revolution, it will be B&N.