I disagree with that. A whole can be less complex then its constituent parts. This is the entire point of abstraction, you can hide irrelevant complexity.
Abstraction is a way to break down a system to fit in our minds & simulations, including for the purpose of designing it, but it's not what a system truly is, and it's not how it behaves. It's a compromise, like for example your calculator has 8 digits, and shows E if you need more, but this doesn't mean math in the universe is limited to 8 digits.
You can't just decide the fact society is made of humans is an irrelevant detail and model them with little rational balls, or whatever you came up with. Because the balls wouldn't do what we do. And I completely disagree it doesn't matter what the human body does. The human body matters to society a great deal, actually. Say, pollution in cities is absorbed by your human body, and affects health, even mental health, and how long you live. This affects social dynamics immensely or doesn't it? We've had entire social movements based on our understanding of the human body, how we categorize in various ways the human body, and violence done against the human body, choices about the human body, how we treat it medically and so on and so on. Countless examples.
Of course you can successfully model specific narrow events, do a representative sample and try to approximately guess, say the outcome of elections. Doesn't always work, but in this very simple example, you can abstract things. But you need to still abstract with deep understanding of what is abstractable for that specific point in space and time.
This approach fails miserably if you need to predict how society will evolve over time, and more fine-grained detail. Say, just 2 years ago society was largely ignorant of the AI revolution bubbling up based on work by Google & subsequently OpenAI. This is something society did, irreducibly, based on the people in it, and their bodies and neurons. And no basic statistics model that abstract us away would have predicted this.
The person you're responding to is saying the abstraction is not the system, much like the map is not the territory, just a metaphor that is easier to work with.
