The only thing there is is individuals transacting with each other. "Society", as an independent entity, is a mirage.

And yes. It probably is like libertarianism. I am a libertarian, and I don't take that as an insult.

Would you agree that the Nash equilibrium is not identically equal to the global utility optimum? If so, then you almost certainly agree that what is best for an individual actor is not necessarily what is best for Society (all people in a political aggregate) at large.

In plain terms, everyone acting selfishly does not necessarily lead to everyone getting what they need.

I have sympathy for your argument, but there's a small note of confusion. Namely, libertarian economists know a ton of game theory. In fact, I only got into game theory due to reading econ texts by writers I later found to be libertarian-leaning. Wouldn't it be a bit strange if the prisoner's dilemma really was a knockdown argument against their views? Maybe there's more than meets the eye?

"Society" is the dynamics that emerge when groups of humans form settlements and organize themselves together for various purposes (food, safety, enjoyment, etc).

Unsurprisingly, these dynamics differ from those found in isolated, individual humans.