It is entirely possible the MOU has been changed again but not posted on the website. The FBI Police operate under the authority granted by the Federal Protective Service, and their MOU/MOA states they have expanded jurisdiction within 300 feet of any federal property.

Another possibility is that the vehicle committed a traffic offense that raises it above a citation into criminal territory. If that's the case, Federal Law Enforcement Officers may choose to use the Assimilative Crimes Act to stop a person or vehicle and effect an arrest for an equivalent State (or in this case District) crime and have it tried in Federal Court.