Price/Performance
The Courier as originally spec'ed would be well over a thousand dollars compared to the iPad's $500. Apple gets phenomenal economies of scale, especially now that they have a supply chain set up for 9" tablets. You can also share all the iPhone and iPod Touch software with the iPad (most apps are Retina display enabled, and they display at near-full resolution on the iPad in 2x mode.)
Besides, the iPad isn't nearly as hard to type on as the article implies. You can almost touch-type with the large 9" screen and auto-correction. It's not like it's one of those 7" devices which would be impossible to do so on.