Just because support for multiple platforms is the goal doesn't mean that it has to share the same code base.

Actually, for the author's simple use case (REST API), writing per-platform would probably have taken as much code as the wrappers themselves. Not to mention improved maintainability and a slew of other factors.

Diving down to C++ for cross-platform libraries starts to make sense when there are 4-5+ platforms (certainly not 2), or if there are complex algorithms (certainly not just a REST API), or if there's a very wide range of functionality (i.e. 100+ API calls).