If I configure the BigQuery source plugin to read from a public dataset but omit the "Temporary Bucket Name" field, then I get an error indicating that it is trying to create the temporary GCS bucket in the dataset project (logs below).
Specifying a value for this field resolves the issue.
Fixed a bug in the BigQuery source that cause automatic bucket creation to fail if the dataset is in a different project.
Fix is in version 0.12.6 of google-plugins