I think they mean that just because code is under github.com/google, it doesn't make it an "official" Google thing.
For example, yapf[0] is under the Google Github org but has the disclaimer:
>Note YAPF is not an official Google product (experimental or otherwise), it is just code that happens to be owned by Google.
libphonenumber doesn't have any similar disclaimer and does seem to be an "official" Google product, but it's hard to tell what Google considers official or not.
Google does not offer (m)any developer facing "official" products. Google offers "official" products for consumers or enterprises (e.g., Gmail, Cloud Platform), but all of their FOSS code, of which there are many, are all "unofficial"; they are FOSS projects that happen to be developed by Google employees but come with a normal zero liability FOSS license (as opposed to their "official" products, which do come with some liability user agreements/contracts).
For example, yapf[0] is under the Google Github org but has the disclaimer:
>Note YAPF is not an official Google product (experimental or otherwise), it is just code that happens to be owned by Google.
libphonenumber doesn't have any similar disclaimer and does seem to be an "official" Google product, but it's hard to tell what Google considers official or not.
[0] https://github.com/google/yapf