OK, two immediate points:

• It’s not a bad thing to implement business logic as a backend system if other clients can hook into it. I know this doesn’t relate to my article, but it needed iterating
• You’re right; phones do have a great deal of processing power. That doesn’t mean, however, that web technologies are the only means of developing cross-platform apps. What about Xamarin, for example? JavaScript would definitely be my first choice, but if I require strong typing then C# is a brilliant option