Of course, Varro was just making a WAG. If you look around enough you'll find all kinds of WackaDoodle Ideas (WDIs) from all eras. Sometimes, as in the case of the measurements by the ancient Greeks of the size of the Earth, they are well-reasoned, other times, also by the ancient Greeks the notion that everything is made of water, they are not. When someone comes up with a WDI that just happens to be fairly close to our present-day understanding we forget that it was just a WDI.