Sorry, that's just inaccurate.
A person with one Jewish grandparent was classified as a mischlinge of the second degree.
While they had some disabilities, they were not actively persecuted, and certainly not treated as Jews.
The author appears to be confusing Nazi race laws with those of the American "one drop" South.
“Based on the bylaws and programs of antisemitic organizations and parties of the late 19th century (as the German Social Party in 1889), the Aryan Paragraph first appeared in the Third Reich in the formulation of the Law for the Restoration of the Professional Civil Service, which was passed on 7 April 1933. It stipulated that only those of Aryan descent, without Jewish parents or grandparents, could be employed in the civil service.”
source: Longerich, Peter (2010). Holocaust: The Nazi Persecution and Murder of the Jews. Oxford; New York: Oxford University Press. ISBN 978-0-19-280436-5.
https://readtiger.com/wkp/en/Aryan_paragraph
I also read Metaxas book on the subject and I believe the author is referring to Pastor Franz Hildebrand, a converted Jew who was ordained.