True…but that is because the the old devs who created Age of Kings and Age of Conquerors were less-caring about historical accuracy than the new devs for Definitive Edition. Many of the new campaigsn reflect more “historical accuracy”.
Some examples of the current devs being more caring about historical accuracy: Edward Longshanks campaign: you are not allowed to build cannon galleons (gunpowder weapons were not developed enough to be small enough to be loaded on ships at that time).
The remade Richard the Lionheart Cyprus campaign prevents no training of cannon galleons too.
And several AI players in various campaign missions have had their civs changed to more closely resemble the history: Slavs in Genghis Khan Mission 4 Goths → Slavs. Normans in Bari Campaign Franks → Sicilians (i.e. Sicilian Normans).
Admittedly, and you are right, the devs have sacrificed some historical accuracy so as to make the gameplay amicable and fun. Barbarossa Mission 4 for instance only saw the Italian city state of Padua be changed to Italians civ, while Verona and Venice remain the same civs as they were originally (the DE devs felt that making them all Italians makes that mission too boring).
The El Cid campaign, when it was first contrived, was meant to advertise the Spanish civ back when it was brand new in AoC. That is why gunpowder units are permitted, as they are the civs best selling points.
So I completely understand on a gameplay level that the devs have–and must–sacrifice some historical accuracy in order to make a enjoyable and pleasant gaming experience.
However, my stance on this matter of Mission 2 Barbarossa is me questioning what would there be to gain from changing the Poland civ from Slavs to Lithuanians?
Like, I get that the Lithuanian civ DOES represent a little bit of medieval Poland, with the access to the Hussar upgrade and the Paladin upgrade, as medieval Poland was famous for its Winged Hussars, and was a heavy-cavalry focused nation. But much of the rest of the Lithuanian tech tree is more Lithuanian than Polish.
It was the Lithuanians, not the Poles, who were excellent skirmishers and make much use of the spear and javelin. Hence why the Lithuanian civ has faster moving Spearmen and Skirmishers, and has a unique Imperial Age tech, Tower Shields, to benefit them.
Moreover, the Poles are actually classified as West Slavs in ethnic terms. So for the AI player “Poland” to be designated “Slavs” is not a total lie. The Lithuanian peoples are not essentially Slavic to begin with: they are a Proto-Baltic people. If you make the Poland AI into Lithuanians, you might very well be offending any Polish players in AoE2 by representing Poland as Lithuania.
Yes…Poland and Lithuania did enter into a personal union to fight together against the Teutonic Knights and other common enemies…but the Lithuanians were not entirely satisfied with the arrangement and how the Poles were “polonizing” the Lithuanian peoples. (It really was a thing: you can look it up Polonization - Wikipedia
While, technically yes you are right that the Boyar is more of a East Slavic unit, i.e. Russian and not much of a West Slavic i.e. Polish unit, it is true that the Poles and the Russians did share a similar aristocracy set up in their respective regions with “Boyar” being a term used somewhat in medieval Poland’s most eastern regions.
But the best reason for why Poland should remain under the Slavic tech tree is because ethnically, it is more correct.