This title is one of Ge Ling Shang's subtitles (Affirmation as Liberation) and is taken from a verse in the Zhuangzi (2/4) which he renders: "Things however peculiar, ridiculous, wondrous and mysterious, they are all One as Dao throughs." I confess that I have yet to understand what precisely he means by "Dao throughs as One". Here is part of his explanation: "For Zhuangzi, One is what Dao is while throughness is the positive state of One." If you find that less than helpful, you are not alone.
Much of what I have read of this book so far reminds me of the Zhuangzi itself; one must dig hard to discover the precious ore that one knows is there. Finding it thereby makes it all the more one's own. Yet, to be frank, opaqueness in this instance is unnecessary, and I can't help but think that Ge, whose first language is not English, has been let down by his editors and publishers (SUNY Press). The numerous glaring grammatical errors are even more indicative of this fact, though they are not so important, except when they make his meanings unclear, which, unfortunately, is sometimes the case. This having been said, there is, I believe, a great deal of significant insight into Zhuangzi to be found here, and I certainly don't fault Ge Ling Sheng for its grammatical shortcomings. [As I have read further, things have improved.]
The more traditional rendering of this phrase is that "Dao connects things into a Unity". Ge, very careful not to fall into the trap of making of Dao an entity that does things, prefers "throughs" to "connects". What I think he means is that Dao is the reality that occasions the unity of things, not what makes them One. He speaks of "throughness" as openness; Dao is the vast openness through which all that happens happens. I am reminded of the identification of qi (vital force) as "an emptiness waiting for the presence of beings" (Chapter 4). It is not a thing, but the potentiality and occasion in which things happen. Words cannot say it; imagination helps, but necessarily falls short.
One precipitate of this understanding of Dao as the openness that occasions the arising of things is the transcendence of the many versus One dualism. Oneness does not subsume the differences of the many, but rather insures them. "One", writes Ge, "is simply a sum of things, that is to say, one is the ten thousand things put together." It therefore, "affirms the existence of every single being. The differences between individuals are not dissolved into some grand and abstracted universal, but are distinguished or characterized by the Oneness of their togetherness." Their oneness is in their differences. (This is very much in accord with the thinking of Guo Xiang, to whom Ge gives a nod as influencing his own thought.)
This is all very profound and profoundly meaningful, and therefore difficult to grasp. Fortunately for you, space is running out and so I won't inflict any more of my own grappling upon you. Instead, I'll close with a paraphrase of what Zhuangzi had to say after a similar discussion with Huizi: "But let's forget all this profound blabbering and just wander together in the blabberless freedom of Dao — shall we?!"
You can check out Scott's writings on Zhuangzi here.