That wasn't the claim. The claim was about the effects of domestication on cognition and at least for dogs it is very obvious they are less intelligent than their non-domesticated counterparts. Same is true for rabbits. So, at the very least, in at least 2 cases it is clear that domestication has lead to a decline in cognitive abilities. It doesn't take much to infer the same is true for people, domestication has reduced selective pressures on cognitive abilities.
If you know of cases and studies where the opposite is true then I'd like to see the research.
If you know of cases and studies where the opposite is true then I'd like to see the research.