Only the frontend is compiled to JS/React. All the logic and state updates stay in Python and are run on the server, so we don't have to transpile arbitrary Python.
From this, it seems like the frontend parts can't have arbitrary Python included and it's the backend parts where you can drop in some Python for the logic. Making this clear early will help people get the right mental model of what this is really doing.
Anyway, good luck with this! I will keep an eye on it for sure.