The main difference I see, is that when you design a web or desktop application on a computer screen, the designer is seeing, more or less, the pixels at the same size than the end user. When you design a mobile app, if you design it on a desktop, and that's where you mostly look at it, you end up having a built-in 2x magnification, and when you finally put it on the device, things are much more scrunched up and less usable than you thought.

The idea in the post is to play with the design, in the device, for a while, and get design feedback much earlier than when you give the app to a developer.