I'll tell you my view more generally of this as a political scientist. I did a lot of cross-national studies. The estimates that are derived about the effects of PR, both negative and positive, are all very subject to case selection, to how we model things, to the exact estimators that are used.
It's a very muddled debate at the academic level, and pointing to a single book and saying this single book is the authority on the matter really misrepresents the diversity of viewpoints in that debate.