Everything that the core stack depended on was written from scratch by Google, there were no third party dependencies with unknown licenses that I ever encounte

Everything that the core stack depended on was written from scratch by Google, there were no third party dependencies with unknown licenses that I ever encountered, or any third party dependencies at all. They started with the STL + POSIX and worked up from there. I'm pretty sure stuff could have been split out. The architecture was sound and the entanglement was overstated. Nothing would have stopped you bringing up a Borg cluster without the global HTTP routing / DDoS / traffic management stuff, as evidenced by the fact that those parts changed regularly without needing synchronized releases of the other parts.