Actually, that's something fundamental, and maybe more thought-trough than appears at first, that they are doing here.
> but my understanding is you start at the foundational idea then move on to the its ramifications and symptoms.
That's certainly how you do in electronics and physics. Explain transistors and logic gates before studying a CPU. We know exactly how transistor behave in some specific conditions. We know they will continue to believe that way when surrounded by millions of others, we know its failures mode, etc...
In economics, you can present the things that way, many textbooks do, but the problem is that it is a lie. You can start with micro-economics, game theory, opportunity costs, marginal costs, supply and demand. The problem is, we do not know how to get from there to macro-economics. It is deeply unsatisfying and an active field of research but it should not be presented to students as a solved problem.
The thing is, if your interest lies in inequalities, you are in the field of macro-economics, you want to take a more observational science approach like, say, astronomy. In astronomy you are looking at stars. Well, you think they are stars, they surely look like stars. You don't know exactly what is inside or how they work, but you start by classifying them into observational categories. You start to have rough models of how they may form. Your models assume a homogeneous non-rotating, perfectly spheric mass, you know these assumptions are wrong so you expect deviations and errors as you refine your model.
That's how macro-economists do: they observe correlations, try to model how they can be linked causally, propose theories, observe if these theories are vindicated (because it is hard to make a controlled experiment in macroeconomics).
So if you are interested in inequalities, start by observing it! Understand the shape it has, how it evolves, and start studying things that seem correlated to it, that it may cause or be caused by.
But that book breaks the lie that we know how to go from micro- top macro- economics. I think it is a very important step. Bring back a bit of humility and clarity to the field.