I didn't say it was restricted to a platform, but that it depends on certain platforms, one of which is PNaCl. asm.js does not depend on the existence of such a platform, since it is backwards compatible with JavaScript.
All the API's available in JavaScript are not accessible from asm.js, you can only use asm.js for numerical code. Emscripten has to wrap all those JavaScript libraries. Pepper makes the need for wrapping obsolete, since you just call directly into the functions provided by the sandbox.
GL is easy because it requires no types. Everything that is required is included in the intersection of JavaScript and C++. More highlevel API's are harder, and that's why we don't see any wrappers for e.g. WebRTC in Emscripten.
