Interesting that it's code owned by Google but a product. Is it because it was developed by someone during work (and hence owned by Google) but nobody from Google endorses it?

This is just how it looks when you publish an open source project at Google.

Google controls the repo hence it's under the Google GitHub org. But then you just slap the "not a Google product" thing at the end to clarify that it's "just" some engineers publishing code rather than the release of the code of a Google product (nor a major strategic open source initiative like Go).

Their process is documented at https://opensource.google/documentation/reference/releasing

So it could even be a pure hobby project - not something done for work - where the initial author (over a decade ago) chose to release it under Google's copyright rather than use the exception process.

Any Googler can write code and open source it on the Google GitHub (within reason, the process is quite straightforward). So no, Google as an entity does not official endorse it, all it means is at least one employee is working on that particular effort.