I don't see how it does. You'll still have to download the language runtime (including a garbage collector for a language like Python). Perhaps it can be cached, but that's a heavy requirement for a web page, particularly on mobile.
By contrast, a language that compiles to regular JavaScript can use the GC built into JavaScript.
WebAssembly is pretty much going to be mapped to Asm.js initially. Most of the features it's getting that asm.js doesn't have yet are future asm.js features which wasm is also to be mapped too. This includes things like pthreads, SIMD and the ability to support GC languages.
By contrast, a language that compiles to regular JavaScript can use the GC built into JavaScript.