There is very little chance a small isolated society could advance this quickly. The invention of the Steam Engine cannot be attributed simply to Thomas Savery and the city of London, it's really the product of all of humanity or at the very least the societies of Eurasia that provided the societal and technical foundation for Savery to build it. From the very first domestication and farming techniques discovered in the fertile crescent to the application of writing, mathematics, physics and all the steps required in between that were shared and built upon until the Steam Engine eventually was discovered. Any endeavor this large leaves a definitive mark on the archaeological, geological, botanical, biological record of the planet. I may be behind on the research but I have seen no indication of this for his hypothesis.