I suspect this is an example of oversimplification; it doesn't make sense to say "1/3 the calories" without including the quantity. The actual meaning is probably closer to "equivalent nutrition from an average American diet would require consuming 3x more calories", which sounds reasonable.

The article mentions that he can raise or lower his weight by simply changing how much of the stuff he drinks, so consuming more calories is simply a matter of drinking an extra glass at "dinner".