Google is too big for the left hand to know what the right hand is doing. Various parts of Google use bionic, newlib, musl, and multiple forks of glibc.

So does that mean all that propaganda about them having a monorepo is bullshit?

Android (bionic), Chrome (forks of glibc and newlib in NaCl), ChromeOS (glibc with patches), and Fuchsia (musl) are all in separate Gerrit repositories. The monorepo for Google proper probably contains all of these, plus the libc used for Google.