When it says, "is ready only to follow the dictates of the subconscious", either it's a bad translation or a very non-Buddhist idea. Using Zen to be a proficient killer, that's a perversion of the teachings. Zen is about becoming a buddha, it is about the perfection of bodhisattva action. Thinking that Zen is good for learning this or that skill is lacking the correct motivation, bodhicitta. It is mistaking Zen for some technique or therapeutic method. Zen is direct insight into the nature of mind. No insight, no Zen. One can use a calm mind to be more efficient in many things, yes. But that's just a calm mind, not Zen.
The quote is a less than ideal translation of that passage.
Takuan's Fudochi Shimmyo Roku
from which it comes does not present Zen as a means to become proficient at killing or any other skill. It is a collection of letters written by him to Yagyu Munenori, a highly placed feudal lord and government official who was also a famous master of swordsmanship. As such, it describes aspects of Zen practice by appealing to experiences that would be familiar to such a person.
For example: in the cited passage Takuan describes the mind's habit to "stop" or "stick" to things. After describing how a skilled swordsman of course does not let his mind "stop" on things like his opponent's sword or movement, his own intention to attack, the rhythm or distance and so on, he concludes: "This I believe is all very well known to you. I only call your attention to it from the Buddhist point of view. In Buddhism, this "stopping mind" is called mayoi
[delusion], hence [the title of this chapter] "The Dwelling Place of Ignorance [Mumyo
i.e. Avidya] and Its Affective Disturbance [Bonno
The subject of the work that follows is exactly direct insight into the nature of mind. It does not discuss cultivation of a "calm" mind. It does discuss original mind [honshin
] and deluded mind [moshin
]. Its thrust is essentially how someone trained as a warrior - in other words, Yagyu and the entire ruling class of Japan then in the early 17th century - could use that background to approach the fundamentals of Zen practice.