They didn't write down the word Indus. It was in the Vedas that we have the word Sindhu. And Sindhu was later changed or mispronounced or just pronounced differently into other words that we now associate with India, words like Hindu, Indus, and India. They all derived from Sindhu, which was the river referred to in the Vedas, and then changed into Hindu, Indus, and things like India. Now, also in the Vedas is the first time that we have reference to a stratified social structure. And we see that with the varnas that are referred to, where at the top you have the, and you could view these as social roles or classes. At the top you have the Brahmins, the priests, the scholars, and the teachers.