While it is true prehistoric man could do geoengineering, this claim is ludicrous. Look at the table of world population growth. At a ratio of 1:1290 and 1:238, there is no way bronze age man (with a good chunk of the population still in the stone age) could cause any real noticeable climate impact when today's (post-)industrial man's impact is still debated. If prehistoric man was the cause of the warming period at the dawn of the bronze age then today's man would have destroyed the world many times over without a doubt.
The problem with the study was that it was conducted by environmental scientists alone who see everything through a human-environment cause and effect. They are taught that nature is in an equilibrium. The radical and violent climate shifts of Earth's past tell us a completely different story.