You're right, it's not very clear, and "essentialism" is a pretty broad term. It is basically the idea that things have essential properties that define and determine them, in addition to "accidental" properties they could easily lose while remaining the same. It is often contrasted with "existentialism," which Sartre summed up as "existence precedes essence."
Since essentialism is often associated with varieties of "idealism," from Plato on, it is indeed confusing to describe the Thucydidean stance of realpolitik as "essentialist." To which objects does this "essentialism" apply? Perhaps power itself? In realpolitik one accepts that power is the "essence" of political relations, the irreducible substrate of the various "accidental" qualities of nations or political players.
This view emerged in reaction to liberal ideals in the 19th century. While those ideals might have discredited the acceptance of "might makes right," they did not suddenly do away with might--or at least that was the realpolitik view. So, as I say, power could be the "essentialist" substrate. Is that what the author means? I have no idea.
We can also think of peoples or nations as having "essential" characteristics, as opposed to liberal universalism. This would mean that they cannot be readily adapted to universal schemes, such as Marxism or Market Neoliberalism, or overarching concepts of globalism and universal human rights. As such, nations exist in mutually exclusive relations between self-interested entities that can only be dealt with pragmatically. Again, just another possibility. Hopefully, the full context will reveal the author's perspective and intent.