So what did the conversion involve? Earth is a C++ application and the faithful Emscripten compiler did the work of converting it to WebAssembly. This is far from just a translation of the C++ to WebAssembly. The compiler also has to convert POSIX calls to something that works in a browser. It also converts OpenGL calls into WebGL. At the end of the day you still need to do some manual work to get the code working correctly.
The Google blog post also suggests what might be next for WebAssembly: