The historical section of this book does a decent job examining how the marriage of Buddhism and Toaism may have taken place. It exposes some of the stories linking Chan to India as probably more legend than fact. However, the author makes no mention whatever of any influence Buddhism may have had on Taoism, or of any of the other quietist philosophies in China. It is very likely that there was interaction between all the traditions for centuries, making it impossible to extricate one from another.
For example, the Mahamadra teaching, Homage to the Coemergent Wisdom, of Vajrayana Buddhism is very similar to Taoism. Take this line: Space has neither color nor shape; it is changeless, it is not tinged by black or white. Likewise, the luminous essence of mind has neither color nor shape; it is not tinged by black or white, virtue or vice. Another line says: Remain in the state of nonmeditation. If you have attained nonattainment, then you have attained mahamudra. Also, the bodhisattva in Mahayana Buddhism is not trying to be good or kind, but is spontaneously compassionate. This is similar to the Toaist spontaneity.
The Heart Sutra which says emptiness is form is common to all Mahayana Buddhist traditions. How is this different from the undifferentiated Tao? The author seems to want to have nothing to do with Buddhism, to put Buddhism in one basket and Taoism in another. This emphasis on separtation doesn't seem very Taoist.
The author also has a negative tone when speaking of Buddhism. He says that the Chinese recognized Buddhism as a simplified form of Taoism. Then he says in another passage, "Simplicity is one of the foremost philosophical impulses that has moved through the long history of Toaism." Simplicity was supposed to be the hallmark of Taoism I thought, but he disparages it in Buddhism.
He also attacks zazen, formal meditation, as evolutionary baggage from the religious Indian Buddhism. (The word religion most likely comes from the Latin word ligare, meaning to bind, and re, meaning again. In Buddhism, the goal is to awaken to the reality of the emptiness of self, and in Taoism, to be one with the undifferentiated Tao, the original source of all things. There really is no difference in the religious aspects of Taoism and Buddhism.) Zazen is evidently not supposed to have been a part of proper Chan. But he quotes Shunryu Suzuki Roshi many times. Suzuki is an ardent supporter of zazen. His lectures in Zen Mind, Beginner's Mind made sense to me only after long hours of zazen and effort. Suzuki says effort is required, but you forget yourself in your effort. Also, Taoism is not entirely devoid of effort. Only after your attainment is there less effort. The author emphasizes the effortlessness of Toaism but totally ignores the practice required. In the book Taoism, John Blofeld gives an account of Taoists using formal meditation in the mountains back in the 1930's.
The emphasis on nonduality is not necessarily a purely Taoist idea. John Blofeld also described Taoist alchemy, the purpose of which was to convert your bodily fluids of the world of dust into a purer form to achieve the ultimate attainment, union with the Tao. This is not a purely nondual appraoch to life. Buddhism is more nondual (LOL) than this.
I think the author short-changes the contribution of Buddhism to Zen. Zen is vigorous because of the blending of the two traditions. The Buddhist component is more introverted, focusing on the illusion of ego, while Taoism is more extroverted, focusing on finding harmony with nature, and ultimately, blending seemlessly with the Way of nature. The combination of the two traditions is what makes Zen unique, but not very different from many other spirtual traditions. This account of Zen is the work of an artist/scholar who does not practice Zen. I get the impression that the author is simply lazy and dislikes effort and guidelines, the Buddhist component as he sees it, preferring flow and ease, the Taoist component. For most, the ultimate attainment of Taoism and the awakening of Buddhism require effort and guidelines, both of which are found in Taoism and Buddhism.