Really not sure I agree with that. Coding is tough, and in my experience developing iOS apps well (and Cocoa apps in general to a lesser extent) is one of the most difficult forms of programming.
And as for marketing and promotion, I'd say even more challenging is coming up with an idea which doesn't need marketing or promotion :)
And as for marketing and promotion, I'd say even more challenging is coming up with an idea which doesn't need marketing or promotion :)