I don't find this analysis very compelling. Sure there is local heterogeneity everywhere at every scale. But the important question is why would humans cross those barriers in the first place? Clearly there is a cost and some were willing to pay it. What's the reward on the other side?
In Eurasia, crossing the desert surrounding the Fertile Cresent would have lead to a myriad of diverse habitats, with optionality in all directions. In North America, crossing the high desert of Mexico from the north puts you into a narrow strip of jungle with no option to go anywhere but through it - a dead end. This is not an insignificant difference!
So while the ecosystems are locally homogeneous as the paper describes, it doesn't touch the core of Diamond's thesis - that the unique geography of the continents afforded entirely different travel opportunities - the reason why humans decided to cross those barriers in the first place.