Not true. Foreign airline pilots are allowed. Must be in CASS only to ride the actual flight deck jumpseat. If not in CASS, but on the reciprocal jumpseat list, may only occupy a seat in the cabin. And without having to purchase a ZED ticket. FOM text says:

9.2.2 Non-Flightdeck Jumpseat Occupants. Pilots and dispatchers from airlines that do not participate in CASS are only authorized to occupy a seat in the cabin. See paragraph 9.10.4 Reciprocal Jumpseat List.

Examples of foreign airlines, whose pilots would be allowed to occupy a seat in the cabin without being in CASS because of being on the reciprocal jumpseat list are Air Canada, Air Jamaica, Bahamas Air, Cayman Airways and WestJet.