Geeze this conclusion seems rather counter intuitive, a much smaller human population's pre-modern agriculture had enough of an impact on climate so much as to prevent an ice age?
As a reference, I’m doing a mapping of Southern Romania’s forrests as they showed up in a 1860s Austrian map (it’s a hobby project that I work on during my spare time) and it’s crazy when comparing it to more modern times. As early as the 1920s (that is in less than 60 years) 60-to-80% of the forrests from the plane areas had disappeared, and that was not because of the introduction of modern technology, because most of the tree-cutting was being done with good old axes, but only because Romania had entered the international agricultural markets and that land was more economically valuable with grain or cows on it compared to trees (the same thing is now happening in Brazil).