Wondering if as an experiment...
1) One were to develop a new programming language, let's call it Greg.
2) One were also to write some excellent, complete documentation of that language.
3) One were to absolutely NOT put any example programs of that language anywhere online. Just don't use the language at all.
If you were then to say Claude for example - "Write me a program to perform task X in the new programming language Greg, here is the language's documentation", I suggest that there is absolutely no chance in hell it would output a viable program
LLMs can't code because they "understand" programming from first principles, they can only code because of the library of code in their training data.