Dear Sechel,
That sefer is an embarrassment. Full of misunderstandings. Shaving was a common practice before the rise of the Catholic Church. The Gemara and Rishonim discuss it in multiple places. There never was a point in time that all Jews had beards. It’s a straight forward sugya until we get to using one sharp blade on specific points of the face. So please don’t use Harry’s.
This is why limud hatorah is a never ending task. It simply won’t do for Jews to make up halacha.