The author may be overstating how rigid they are. iOS apps can be much less dynamic than web apps, mostly because you're working at a "lower" level in the GUI stack. Instead of having a layout engine that recalculates the positions of your UI elements for you (eg, HTML and the box model), you mostly have to manage the position and sizes of everything yourself.
You could create your own layout engine, but most developers choose not to, especially since the consistency of iOS hardware means they can predict screen sizes easily. Your other option is to make your app a thin wrapper around a Webkit view, but that tends to produce poor results (see the Netflix iOS app for an example).
You could create your own layout engine, but most developers choose not to, especially since the consistency of iOS hardware means they can predict screen sizes easily. Your other option is to make your app a thin wrapper around a Webkit view, but that tends to produce poor results (see the Netflix iOS app for an example).