You realize there area acids naturally occurring as well as added to a lot of food products.
I don't know how much is in Coca-Cola, but chances are it is only there to attain overall desired pH level of the product.
I've been around food processing and they do add acids or bases to adjust pH level of product as needed all the time. Sometimes it isn't how much acid is necessary in the formulas but more of a situation of keep adding until certain pH level is reached.
That one cup of acid that may be deadly if you ingest it is harmless if diluted into hundreds or thousands of gallons of product.
Vinegar is an acid, takes a lot more of what you buy on grocery store shelf to do same job as a 50% solution of phosphoric acid when adding it to a large batch of product.