This "secret codes" thing will never die.
As someone kindly pointed out, the British has at least one ship slammed by an Exocet after the "secret codes" were supposedly delivered.
Secret Codes is newspaper-speak for classified weaknesses; it is a lot easier for naive journalists to say secret codes, as if all you need is a radio that transmits some digital burst, and back-door software in the missile receives this and says "Guess I'll shut down now." It does not work that way.
The world arms industry is probably trillions of $$. If a nation inserted magic codes into its weapons, all it would take is ONE instance, and that nation would forever lose all of its business, as no one is going to buy a weapon that can be turned off.
Weapons are designed to be extremely robust and as immune as possible to external influences.
So the French didn't say this - "Transmit 'XYZ12345' on 212.25 MHz and the missile will turn off." More likely they said "The exocet is vulnerable to the following decoys and defenses..."
Do people really think a missile as simple (and it really is quite simple) as an AIM-9L has a miniature radio receiver in it? It has a quartz IR window, and the proximity fuse uses a laser. There is no RF (radio frequency) anywhere in it, and nations who own them can take them apart and identify every single component. No radios. You point an AIM-9 at a heat source, it'll guide and explode.
