Libraries internal to Google are kept at the latest and consumers are updated to use the latest APIs. 3rd party libraries are checking into the monorepo at a specific version and everyone uses the same version.
Inside Google's main repo there are different build targets for libraries with incompatible API changes that are too difficult to fix all at once, e.g. there might be numpy_1_8 and numpy_1_10 separately.
Python at Google muddled along for years without numpy at all so it's not like anyone would be seriously harmed by having an old release in the repo.