Does a "subpoena" mean a judge was involved? The post says the subpoena was issued by the DOJ.
Edit
Even that is technically wrong; some DOJ subpoenas are apparently preauthorized by statute.
https://www.justice.gov/archive/olp/rpt_to_congress.htm
tl;dr: Everyone from the Appalachian Regional Commission on down can subpoena you without a court being involved. And of course Congress has inherent subpoena powers.
This is a step towards answering my noodly question earlier in the thread: authorization for NDAs and "gag orders" in subpoenas appears to be controlled by (varying) statutes.