I was quite surprised to see that, too.
It makes about as much sense as thinking that Russian military of the Imperial period was all about drowning the enemy in the bodies of your infantry. But that's what we have in ours NIs and we have to play with that in mind. Same gonna happen with culture.
Speaking from a pure gameplay perspective, though, for me that change means just few more cheap provinces to culture convert, as I obviously won't tolerate "Novgorodian" or "Ryazanian" culture nonsense, just like I don't tolerate "Belorussian" or "Ruthenian". And I'm entitled to my own views. As I see it, the only difference between Muscovy and Novgorod in "culture" of the relevant timeframe is that Novgorodian lands lay further to the north and were thus less affected by the invasion of the Tatar barbarians 200 year prior the game events. Hardly a solid justification to stick to a theory of significant cultural difference.