LibKGAPI does a sanity check on the data that was received from the Google API to make sure it contains the expected kind of resources.
In src/drive/parentreference.cpp the "kind" string that the code looks for was "kind#parentList" rather than "drive#parentList" as actually returned by the Google API. Therefore, further data processing was aborted and the class would always return an empty ParentReferencesList result. After correcting this I was able to fetch the desired non-empty output.
I looked for similar occurrences elsewhere in the code and found that src/drive/app.cpp was affected by the same problem.
References:
https://phabricator.kde.org/T3443
https://developers.google.com/drive/v2/reference/parents/list
https://developers.google.com/drive/v2/reference/apps