He claims to have learned Japanese at University but other than that has no academic credentials. Furthermore, he admits he doesn't have a strong grasp of Japanese but maintains he can read Hiragana and Katakana.

This is exactly the level of proficiency with Japanese at which the only excuse for neglecting Japanese primary sources is an unwillingness to put in the time to understand them. I would know, because I'm at exactly that level of proficiency myself.

Despite the writing system being modeled on Chinese, Japanese is not like Chinese with its notoriously difficult grammar. Once you've studied Japanese at a university level and know hiragana and katakana, you can (or at least should be able to) parse a formal Japanese sentence. Casual writing may be more difficult, but how convenient that government and academic sources are written formally! For crying out loud, the hiragana parts of sentences are almost entirely there to depict how things function grammatically! A sentence actually tells you how to read it as you're reading it!

All you have to do at that point is look up the kanji you don't know-- and unless he only took one very introductory class in Japanese, he should have at least 100 common kanji under his belt already. Sure, this can be a time-consuming and laborious process, but having access to basically any device with a touch screen makes it a lot less laborious than it's ever been; instead of looking up unfamiliar kanji by stroke count (which is easily possible-- though admittedly time-consuming-- even for a novice, because surprise surprise, the Japanese people have developed some nice structured methods of looking things up in their language), you can draw them on your phone or even game system (I have a kanji dictionary for Nintendo DS) and immediately pull up a list of meanings, pronunciations, and words the character is used in. It's not much harder than just looking up an unfamiliar word.

I can understand not wanting to make this level of time commitment to read Japanese sources if you've only got a casual interest in Japanese history or are going for a non-authoritative grasp of it academically, but at the point where you're not just writing a book, but writing a book that directly challenges currently existing scholarship, there is really no excuse for remaining that ignorant except that you want to.