Your argument fails where it equates someone who only codes in one language to an LLM who is usually trained in many languages.

In my experience, a software engineer knows how to program and has experience in multiple languages. Someone with that level of experience tends to pick up new languages very quickly because they can apply the same abstract concepts and algorithms.

If an LLM that has a similar (or broader) data set of languages cannot generalise to an unknown language, then it stands to reason that it is indeed only capable of reproducing what’s already in its training data.