The "runtime" is a google internal distribution of libc + binutils that is used for linking binaries within the monolithic repo, "google3".
This decoupling of system libraries from the OS itself is necessary because it otherwise becomes unmanageable to ensure "google3 binaries" remain runnable on both workstations and production servers. Workstations and servers each have their own Linux distributions, and each also needs to change over time.
IIRC Google has a policy whereby all google3 binaries must be rebuilt within a 6-month window. This allows teams to age-out support for old versions of things, including glibc. grte supports having multiple multiple versions of itself installed side-by-side to allow for transition periods ("v5" in the article).
This decoupling of system libraries from the OS itself is necessary because it otherwise becomes unmanageable to ensure "google3 binaries" remain runnable on both workstations and production servers. Workstations and servers each have their own Linux distributions, and each also needs to change over time.