LLMs take natural language as input and produces it as output (note that producing source code is the same thing). The algorithm that takes input and produces output is still written in mathematical precise symbols and needs to be accurate, even if the input and output aren't expressed in a formal language.

We will see if and when an algorithm can be parsed and executed from a natural language "source code" and if that is an improvement.

Also note that "source code" implies it is a code. Natural languages are not a code, that is, a unique mapping from a set to another.