True. And I agree it seems like bullshit. Now consider the entirety of economics.
Have you studied economics?
Judging by the outcomes of the field and the absurd world we live in today with insane boom/bust cycles every few years, sky-high levels of wealth accumulation, inflation out of control world-wide, one cannot wonder how nobody seems to understand it.
From the outside, it looks like either they are producing incorrect/incomplete theories that simply don't work in the real world, or it's all so theoretical that it never really translates into people's lives.
Or, alternatively, the system is working exactly as intended and the goal was always to maximize for a few individuals and not society in the first place.
Empiricism is vulnerable to the changing awareness of the participants: if you discover a reliable principle it will be manipulated until it is learned. Once it is learned it is no longer reliable.
Rational agents are typically poor proxies for human behavior, and the more of them you have the worse the narrative to make it even seem that they could plausible be the way humans behave.
I'll try to answer your question through the lens of that book: the models that economists develop work under the models assumptions, and therefore they quite rarely fit well extremely complex phenomena like inflation; in general, economists treat the problems they face on a case per case basis by comparing different models outcomes', looking at the historical data, discussing the merits of the respective arguments for one or the other policy, and obviously depending on the specific goals you find more desirable (equality vs growth for example). The way the economy is ran is not necessarily representative of the economists consensus, or even that of the economists that are employed by the biggest policymakers, as the policies chosen might be the ones deemed more politically feasible, or more desirable for the particular politician's interests, and not the ones that are economically most sound and well-founded.
By the way, a topic I've been always fascinated by but never inquired further is that of social choice, wherein the economists analyse the incentives of classes such as that of policymakers.
Their assumptions don't hold. Assume people make perfect decisions, assume they all act within their own self-interest, assume perfect information, assume effects to the environment don't matter, neither do human lives. Assume acquiring money is the only goal anybody has.
I love how much these horrible assumptions shape the economic policy of the world.