This book looks like a bunch of Malarkey. As a person with Scottish descent and family still in Scotland... and somewhat well read on the subject... I don't believe for a second there is a major Jewish contingent in the blood.
By logical extension: It would be a kin to saying that since many Americans have Scottish decent, that America is Jewish.
I am willing to bet this book glosses over and selectively cherry picks the cultural History of Scotland... the difference between Highland and lowland culture....clan history going back before the Norman invasions to reaffirm the hypothesis. The fact that Scotland remained Catholic for a lot longer than England for instance?
While I haven't read the book, this seems like reaching for something and not credible as I would want to investigate the tenacity of their primary sources.