Does Zamzam water completely substitute food if a person only drinks it, given the Prophet's (peace be upon him) saying, "Indeed, it is blessed; it is food that nourishes," or is that specific to certain people, and who are they?

The virtue of Zamzam water is for those whose truthfulness is sound and whose intention is pure, like Abu Dharr (may Allah be pleased with him). It is not for everyone. And Zamzam water is for what it is drunk for. And Zamzam water is the best water on the face of the Earth.