Google operates across so many verticals that it's difficult to argue a side project is outside the scope of Google’s business and therefore Google could argue it has copyright over the work. To make it easier for engineers to keep contributing to open source, there’s a fairly straightforward path to release code through a Google-owned repository (if you look at github.com/google it is full of personal projects alongside official ones).

There is an official process where an engineer can apply to a committee to have Google waive any copyright claim. That requires additional work so if your goal is simply to publish the code as open source and you do not mind it living under the Google org, using the Google repo path is usually much faster.

Disclaimer: ex-googler, not a lawyer, not arguing whether or not the situation with copyright assignment is legally enforceable or not/good or bad/etc.