I think we still need an LLM to enable the system as a whole to understand vague and half-baked human input.

I can easily ask an LLM to write be a function in a random programming language, then feed the output to a compiler, and pipe errors from the compiler back to the LLM.

What doesn't work so well is typing "pong in java" into a bash shell.

This isn't a perfect solution (not even for small projects), but it does demonstrate that automated validation can improve the output.