They aren’t directly enforcing EPA’s rules. They’re enforcing customs rules.
If the vehicles entered this country illegally it’s a customs issue. DHS’s ICE (Immigration and Customs Enforcement) component has appropriate and proper jurisdiction.
If only they were half as enthusiastic about enforcing immigration law.
They may have jurisdiction, but good grief! Who cares if 40 Land Rovers are gathering dust in American garages. Don’t they have anything more important to look into? And couldn’t one cop and a tow truck handle this just as well as a caravan of DHS vehicles?