Even if someone observing the flow of incoming and outgoing trucks could determine the ingredients and their precise proportions, it's the cooking method that produces the right taste, and someone who counts trucks won't be able to figure that out. You can sniff packets in my LAN all day long, but that won't tell you 100% of things that go on inside my computer. Tricking me to install spyware on my computer, on the other hand, is a completely different matter. If the Coca-Cola formula is known, it'll be because of corporate espionage and not something you can tell from publicly available data.
Indeed, I suspect it's pure marketing hyperbole. Even if only two humans currently know, the machines must know to mass-produce it. Their best-kept out-in-the-open secret in my opinion is the way they are involved (as are others) with things like murdered union leaders and generally poor living standards in places like Colombia and other nations.
The company sells enough concentrate to produce millions of litters world-wide yearly.
Surely there are hundreds of trucks entering and leaving the facility, and by now the ingredients and their dosages are quite know.