Maybe the term "Imperialist" is what breaks it, but until the past two centuries the most common nation by a long shot was that of a monarchy, with a ruling class who's job was to be the elites of the military. Expansionists, or rather Imperialists, where actually uncommon until Renaissance Europe, but many of these monarchies and especially empires had to insist they had a right to rule over extensive lands or different cultural groups, which is pretty much the exact description of the Hegemonic Imperialist. Once reaching a point where administration became difficult, they usually stopped trying to expand and focused on internal stability, even though their mindsets barely changed.This is a huge claim that needs a scientific proof.
Especially when there are so many misconceptions about wars and violence in general before the 18th century. Of course if you base your knowledge of history on strategy games you would think so - but historically it's simply false. Most people just want to live their lifes, and states will fight for ressources, not for hegemony. Hegemonist empires are severely overrepresented because they look bigger on maps and because they have more ressources to pass on their legacy. But behind every Roman Empire, there are the countless city-states, tribes, kingdoms and proto-states they conquered. Mathematically, there's necessarily more non-imperialist states than imperialist states in history.
This doesn't mean I agree with Paradox lumping so many types of nations into one, mind you. I don't think in any universe the Mongols (Barbaric Despoiler) should be lumped in with the Roman Empire (take your pick) and Warhammer 40k's Imperium (Imperial Cult). They should absolutely break them up into more interesting types. I'm just saying WITH the definition as they've described it AND relative to other options, Hegemonic Imperialists are "technically" the most dominate and common nation in history, although yes one can debate how prevailant they really are since history doesn't record minor nations quite as well (history is written by the victors after all).