This assumes that their technology would be similar to ours. They could have used biological mechanical parts, made from plant or animal material. They could have had radically different theories of mechanics or construction that better fit the state of the planet at their time.
In general the statement "They couldn't have been civilized because they aren't exactly like us" limits what we look for and how we look for it.
In general the statement "They couldn't have been civilized because they aren't exactly like us" limits what we look for and how we look for it.