That was a joke, right? Because if you really believe that, you need to read some more history.
This is a quote from the Internet Encyclopedia of Ukraine, an academic publication of the University of Toronto (which has one of the best Slavic Studies departments in North America):
Link:
http://www.encyclopediaofukraine.com/display.asp?linkpath=pages\R\U\Ruthenians.htm
There is nothing in there about Polonization, other than that by the 16th Century, they lived within the borders of the Polish-Lithuanian Commonwealth. Polonization during that time was directed almost entirely at the upper echelons of the nobility. The burghers, peasants, Cossacks, poor nobles and other estates (and thus the vast majority of the population) remained firmly Ruthenian. The myth that Ukrainians and Belarusians were somehow split apart from the "true" Russians (Muscovites) is just that, a myth. Cultural differences were age-old, thus there was really nothing to split. Furthermore, the name "Ruthenia" clearly was used at the time, as was the word "rusyn" from which it was derived. Ruthenians *did* appear in the time period. I'd also like to point out that Slavs living in the southern principalities (corresponding to modern day Ukraine) referred to themselves as "rusyny" while those in the North-east (Russia) referred to themselves as "russkie", so clearly medieval Slavs were aware of national-cultural differences between each other, even within Kyivan Rus'. However, I think it would not make sense to include Ruthenian as a separate culture, at least not at at 867 (where I think lots of tribal cultures should exist) or 1066 (imagine how weird it would be for some of Yaroslav's sons to be one culture, others another....)
As for Ruthenia, I think it should remain a kingdom. Danylo Romanovych was crowned King of Rus' by a papal legate in 1253, so historically there was a Kingdom of Ruthenia (often referred to as Halych-Volhynia). The k_ruthenia coat of arms is the coat of arms of Halych-Volhynia, so it is really not that much of a stretch. There was never actually a King of Kyiv, nor a King of Novgorod, certainly not of Suzdal, so the current de jure kingdoms are not horrific (I think their borders could be changed though). Certainly not any less realistic than some of the other Kingdoms or Empires in the game (here's looking at you, Carpathia).
I'm not sure what you mean. Are you referring to the face-packs? Or to how actual true-to-life Slavs and Scandinavians appear?