Nondeterminism is not the issue here. Today's LLMs are not "round trip" tools. It's not like a compiler where you can edit a source file from 1975, recompile, and the binary does what 75'bin did plus your edit.

Rather, it's more like having an employee in 1975, asking them to write you a program to do something. Then time-machine to the present day and you want that program enhanced somehow. You're going to summon your 2026 intern and tell them that you have this old program from 1975 that you need updated. That person is going to look at the program's code, your notes on what you need added, and probably some of their own "training data" on programming in general. Then they're going to edit the program.

Note that in no case did you ask for the program to be completely re-written from scratch based on the original spec plus some add-ons. Same for the human as for the LLM.