The article defines "the tablet market" narrowly and ignores important competition for the iPad in terms of sales (if not mindshare). The iPad competes with devices such as the Kindle which have significant adoption and handle the killer slate app - reading very well at very competitive price points. This class of competitor also often provides better performance in important areas, e.g. battery life.

I also suspect that if people were asked their preference regarding wide screen format vs. 4:3 the iPad would not fare quite so well as it does in questions regarding screen size.