I think there's some confusion in your post. You're seeing people claiming that the rate their code changes is so fast that they find it hard to keep their mental model up to date and in sync with the actual codebase. They frame this as a LLM issue, but those of us who worked in large teams will understand how it feels to be faced with a dozen changes popping up in critical areas of a code base each time we prepare to submit our own code change.
Once you understand this, you'll easily understand where all the efficiency gains are manifesting. Today's lone cowboy developers are experiencing the same type of struggles to keep up with a project that in the recent past affected projects worked on by large teams.
I mean, think about it for a second: what do you think is behind this higher rate of change?