> Specifically, I think they are less concerned with (say) specific Excel code leaking than with the knock on effects of a cheap perfect substitute.

Is there any evidence that an LLM could actually generate a perfect substitute for excel solely through prompting if only the excel source was in the training data? I hypothesize that designing a prompt for an LLM that captures all of Excel's properties would be comparable in difficulty to reimplementing the functionality without an LLM.