“How exactly would a federal court or any federal official "expunge" a state court conviction? Seems a bit made up to me.”

After obtaining a state expungement, you can contact your State Repository to petition the FBI to comply with the state order. Once (or if) the FBI complies with the petition, the State Repository will then issue a letter of compliance, which will serve as proof of federal “expungement”. No records will then be visible as a result.