Disneyland is part of the U.S.-Mexico border?

According to federal codes and regulations, it is.

Customs and Border Patrol agents searching for aliens can board any plane, vehicle, railway car, or conveyance a “reasonable distance” from the border.

The Fourth Amendment to the U.S. Constitution typically requires a warrant for such searches, but an exception is written into federal law. (See here.)

To find out how far that is is you have to look at US Code of Federal Regulations. 

Turns out a “reasonable distance” is within 100 “air miles” from any external boundary of the United States.

That includes Huntington Beach, Newport Beach, Irvine, Costa Mesa and half of Catalina Island.