It is true that the author failed to express himself very clearly, but what he says is that the ancients did not use an abstract unknown denoted by an arbitrary symbol like "x", but they used instead of it the length of some line segment, instead of the squared value of the unknown, the area of some square constructed on that segment, instead of the cubed value of the unknown, the volume of a cube constructed on that segment, transforming thus an abstract algebra problem into a concrete geometry problem, which then they solved.
While that excerpt may not be the best example of mathematical writing, the only conclusion that I can take from it is not that the book must be bad, it might be good, but that the book is written at a very elementary level, so it should be good for children or at most teenagers, but it does not seem to provide any information or idea that would be unfamiliar to someone already experienced in mathematics.
As a child, I have read many books of "popular" mathematics, some of which have made a lasting impression, and perhaps this book is as good as those, taking into account other texts written by Terence Tao, which I have read. Thus the book might be good for a child/teenager who likes mathematics.