Not every optimisation is the same. LLMs can optimize code that is behind known practices, but won't usually invent new data structures or algorithms to solve a unique problem.
While it's unlikely for LLMs to implement patterns not present in their training, current training techniques absolutely can produce practices and data structures and algorithms that humans have never done before. Modern LLMs are not just trained on text. They are trained on computer use, including - at a high level - solving programming problems with code. They are not just rewarded for producing the most likely next token. They are rewarded for producing tokens that solve problems. Patterns and techniques can emerge from this that are not present in any human behavior.
They are rewarded for code compiling, thats about it. Thats why the code is terrible. Ive yet to look at a project made by an llm whos user swore this "frontier model is the one", where the code wasnt utter garbage. The heavy llm users dont care as long as it compiles.