There is nothing wrong with most developers working on and communicating about the entire code base. Having teams work in silos is not a benefit. You're touting as a benefit what is one of microservices' gravest issues - teams stop communicating beyond the surface level of their respective APIs.
Have you tried coordinating entire teams to work on a shared codebase?
Honestly, I have never been in an organization so large that this became a necessity (if you solve tens of different problems, that would require almost thousands of developers). But coordinating single developers without an API is hard enough already, I can only assume for teams its nearly impossible.
Define “codebase”? You can have multiple services, user facing apps or modules inside a single repository, but if there are no boundaries coordination will be difficult of course.
The definition implied by the GGP is: shared codebase = everybody will change the same lines; separated codebase = people will work on different sides of an API.
At least, that's what I understand from his comment.
