Plomin's central argument seems to be that once you control for environment, genetics explains the remaining differences:

"Plomin’s argument is that, in a society with universal education, the greatest part of the variation in learning abilities is accounted for by genetics...

...Thus, the average height of northern European males has increased by more than 15cm in the past two centuries. That is obviously due to changes in environment. However, the variation in height between northern European males is down to genetics"

To conclude that it is "nature not nurture after all" based on this argument, seems quite backwards.