Without knowing more than a little bit about the history of Western harmony and its scales, I am comfortable saying you should not look to this author for history lessons.
The reason I write that is because he wrote the words "white" and "Aryan" as synonyms in 1913 which highly suggests his history is racist. There are other aspects of the history he wrote that could be at the very least culturally insensitive, but the use of "Aryan" in this way is too highly suggestive of outright racism and white superiority.
Yes, 1913 is before the Nazis came to power or even prominence, but the history of the word "Aryan" is telling in this case. Originally, it was a "self-designation by ancient Indo-Iranian peoples" (ironically). The word was only adopted as a racial category in the 19th century by an author who believed that "blonde northern European 'Aryans' ... had migrated across the world and founded all major civilizations, before being degraded through racial mixture with local populations". The author, Arthur de Gobineau, did influence the Nazis with his thinking and theories of white superiority.
That's not to say that racists cannot be good historians at the same time, but it's clear that this particular history is laden with racism and therefore cannot be trusted on its face to be accurate or complete. How could anyone feel confident that important details of the history of scales have not been changed or left out because they do not support a worldview of white, "Aryan" supremacy?