[air]

It's discussed in the article:

Many people believe that the source of this myth was a 1945 Food and Nutrition Board recommendation that said people need about 2.5 liters of water a day. But they ignored the sentence that followed closely behind. It read, “Most of this quantity is contained in prepared foods.”

Indeed, from the references of your link:

The Panel has decided that the reference values for total water intake should include water from drinking water, beverages of all kind, and from food moisture.