You are kinda correct... in that the support is very weak at this time....
at ton of 3rd party modules I tend to install with pip would likely not work... and the feature set and edge cases in creating the bytecode are more rampant than I'd like...
Not counting manually transpiling into C or C++....
... or counting introspection madness in something like this disassembly tool
https://github.com/athre0z/wasm
You got these at least I have seen a few others somewhere might have tried them don't remember...
PyPy.js as WASM bytecode
or
Compile Python -> LLVM -> asm.js -> binary.wasm
It should be noted all the WASM stuff is still fairly new... and I have yet to see a pure WASM VM for Python