I like your point about having designers a the particular platforms UI tool/system. It's sad though that they can't learn a platform agnostic tool so that skills are more shared and portable across platforms. So I like this kind of solution for tackling that. Also, making it easy for designers to mockup interactions in more than just series static mockups is much better for articulating their designs. This is why so many design prototyping tools are coming out of Facebook, Apple, etc these days. Having it online, collaborative, and platform agnostic seems like a strong basis for a successful production provided they can provide tooling to make it easy to use on popular platforms. I think an embeddable UI runtime you can drop into projects would be the ideal technical approach.

To be fair those tools only existed very recently, at least on Android. And it's only very recently become good enough for anyone sane to use.