Yep. They could also use some shared C/C++ bits to generate various drawing metadata ("the model") and delegate the rendering/input handling to whatever platfor

Yep. They could also use some shared C/C++ bits to generate various drawing metadata ("the model") and delegate the rendering/input handling to whatever platform-specific APIs. It may be more work, but if you're not going to build an actual native app, why bother? These apps are built as a sales point ("look, we have a Mac app!"), without the customer in mind.