"Former Googlers" were probably used to using protobuf so they could get from a function call straight out to a struct of the right schema. It's one level of abstraction higher and near-universal in Google, especially in internal-to-internal communication edges.

I don't think it's a strong hiring signal if they weren't already familiar with APIs for (de)serialization in between, because if they're worth anything then they'll just pick that up from documentation and be done with it.