our dukes that 'have united' and 'led' the 'various peoples' within our borders to greatness
The latter half of the sentence that you didn't mention.
That's fluff.
Compare it with the description/effects of the Rajput Kingdom:
Description: "A Rajput kingdom is ruled through
close ties of
loyalty and
kinship with the
clans of the ruling dynasty
and their retainers. As the head of a Rajput clan we have a long and strong tradition of martial prowess that we can rely on. Whenever war or
disaster comes near we can call upon our supporters both from within the kingdom
and in nearby regions."
The effect:
+0.25 Yearly army tradition
Going through what you bolded.
"United"
The dukes have united Lithuania. Past tense. This has happened. Lithuania is one tag, the southern parts aren't subjects. And the grand duchy provides extra governing capacity, the lack of which is normally an incentive to not annex subjects. So that works out nicely.
"Led"
I did, in fact, mention that "Lithuania was
led by dukes."
"Various peoples"
Lithuania gets +3 tolerance of heretics in its traditions. I'm fairly sure it starts with the other two main cultures in its lands as accepted, too.
"Greatness"
What would that mean as a modifer? And I don't think it would be prestige; The only tier 1 government reform that gives prestige is the Celestial Monarchy, and that's only +0.5/year.
The description you're quoting is not a historical analysis of the nature of the governmental structure of the Grand Duchy of Lithuania, the consquences it had on the state, and how that should translate into the game. It's bit of in-character flavour text, that was added to explain why the Duke of Lithuania is a duke when country ranks were added back in 2015.