"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.

The question evaluates different skills when you solve it in Java. If you allowed XML, you'd see Java candidates reach for the standard library, as it has a built-in XML parser. Using plain text responses was a good fix, as the candidate can focus on concurrency, networking, and error handling, which is probably what you were trying to assess.

In my experience it’s Googler’s not Java people. All of my friends at Google tell me how everything is already set up for them.

You mention it was 8 years ago, at that point a typical Java dev would be already using Spring Boot for requests and deserializing JSON to POJOs (with Jackson under the hood).