In this particular section, the authors already took good care to separate internal rules from general advice: "On their face, the benefits of using exceptions outweigh the costs, especially in new projects. … Things would probably be different if we had to do it all over again from scratch."

You are correct and I did not overlook that part.

Also as stated in the guidelines, some of the reasons are the intention of integrating the open source projects with an internal Google C++ code base that does not use exceptions. So integration would be difficult.

However, the question is how come such a large code base using C++ without Exceptions come to be. Specially on a company with so many employees chairs of C++ committees :-)

Just to make it clear, I prefer the C and Rust model around error handling. I will also concede that, for GUI applications and when on a coherent code base, you probably want C++ Exceptions.