I've actually never heard of diffusion for planning. Do you have a reference?
A diffusion model for text generation (like Diffusion-LM) still has the training objective to produce text from the training distribution, optimizing over only the current episode - in this case a short text.
The strategies are manually reviewed with clear prompt injection attempts rejected.
I think the program approach proved unworkable. It is simply too difficult to write a program that can analyze another program effectively when the program being analyzed has to be complex enough to do its own analysis of other programs.