If you did it in a loop until the test passed, maybe?

Your result is essentially impossible without the original. With ffmpeg, your result does not depend on ffmpeg specifically - you can use any video creation tool.

Repetition isn't really a factor in deciding whether something is infringing or not - check the copyright law in your jurisdiction. Here if you look specifically at what an LLM's sampling stage is doing, it's choosing non infringing tokens (i.e. rust source code) over infringing ones (i.e. C source code). So it's making an intentional choice to do something similar rather than creating something that has the same expression. That doesn't seem like it's copyright infringement to me.