100x is a bold statement.

You can easily get to 100x in a greenfield project but you will never get to 100x in a legacy codebase.

That depends on the code-base. I've found that hand-writing the first 50% of the code base actually makes adding new features somewhat easier because the context/shape of the idea is starting to come into focus. The LLM can take what exists and extrapolate on it.