You might have disproved the point that all mammals very less than 10 Kg, but Wikipedia states on the Coryphodon[0] that the size of the species also decreased,

You might have disproved the point that all mammals very less than 10 Kg, but Wikipedia states on the Coryphodon[0] that the size of the species also decreased, and that "these changes in size are thought to be linked to global climate change". So the general argument certainly seems to hold. It also "had one of the smallest brain/body ratios of any mammal, living or extinct" so that may not be the best example of how this kind of environment might be fit for humans. [0]https://en.wikipedia.org/wiki/Coryphodon https://en.wikipedia.org/wiki/Coryphodon