Yeah, you're probably right. I was going largely for shock value.

The Fremen are shown to have some farming down in the southern regions of Dune which they keep hidden by bribing the Spacing Guild or whatever to not put satellites in orbit that can detect their crops. And the Harkonnens are too greedy and stupid to care. How much of it could actually meet their needs is unknown, but the issues of dry farming and desert ecology is one of the main themes in the first book.

Of course, by the third book, Arrakis has been sufficiently terraformed to allow much more agriculture, but the original question seems to be how the lived prior to the events of the first book.