Nations (in the western definition of the term) didn’t exist before the French Revolution.

Not sure I agree. China did by the Qin Dynasty. They have spent centuries defining the notion of centralized government right down to civil service exams (imperial examination system) for a thousand years. Rome was a pretty well-defined system of government, no?

I suspect the China issue was a carve out in the comment you replied too. Since they were referring to the west.

As for Rome, it was never a Nation in the modern sense. It started as a city state and became an empire.

My understanding is that nation states (where the rule and the border were defined nationality of the population) were born as a reaction against empire. Starting with the French Revolution, where the desire for the French to be ruled by French in France was new concept.

Thanks. A way to frame history I never encountered.

That really wasn't new. The Imperial Senate of the HRE was assigning rulers to fiefdoms also based on locality.

He's using the term "nation state" in the political science sense, not in the colloquial sense of a political entity that encompasses a people (nation). The very concept of a modern nation-state is, as tomcam stated, quite new. Before that, loyalty was primarily given to people (individuals or families) rather the state as an abstract concept. Empires like Rome touched on the concept as far as we can tell from historical sources, it was fairly different from the concepts of patriotism today.

The concept of "modern” nation-state is modern and therefore new, it cannot be otherwise.

The original comment remarked on nation-states, and that they have existed in the past. They are not new as far as I am aware.