Now the question is: did you gain the same knowledge and proficiency in the codebase that you would've gained organically?

I find that when working with an LLM the difference in knowledge is the same as learning a new language. Learning to understanding another language is easier than learning to speak another language.

It's like my knowledge of C++. I can read it, and I can make modifications of existing files. But writing something from scratch without a template? That's a lot harder.