I read the sample chapter: it's surprisingly bad! (He's a good writer normally.)

It's boring, and badly-written (the Greeks didn't use symbols like x, instead they used x), with poorly-chosen examples (curtains not a good example of abstraction; correlated risks a good example of correlation, which is off-topic here, but a bad example for algebra), and lacking pictures that would help (the wine barrel).

Your quotation is taken out of context, so it has become completely meaningless, but the original text is not so bad as your weird quotation implies.

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.