Ignoring the gaping holes in the engineering side of the argument (because clearly there is you use the AJAX "language" to make calls to a magic blackbox that handles all of the data storage, notifications, business logic, etc), the UI/UX arguments aren't even fully baked.

For mobile applications he recommends developing a mobile website instead of a native app. The biggest downfall of mobile websites is that iphone and android have rather different vocabularies for user interactions (hardware buttons vs. all touch screen) and what makes sense on one platform often is confusing on the other. That's probably the biggest downfall I've seen to using a framework like PhoneGap. Yes, you can quickly mock things up to try (which is great for iterating on the UI/UX aspects) but there either end up being two versions (sharing some common code for logic/backend interface) with different UIs built on PhoneGap or you go ahead and build native apps to fully fit into the expected user experiences from both platforms.

Is it silly to ignore UI/UX, of course, but expecting the backend to just magically work (and work well) is just as unwise.