No, they are not, they follow the same logic as all other cultures in the region. Severian culture is not there due to the Severian tribe itself, but due to the name of that land, that historically comes from the name of the tribe.
The cultural division is based on the historical political division of the Kyivan Rus, based on its biggest and the most stable duchies that had existed for several centuries. According to
this book, there is a strong suggestion that East Slavic identity evolved around the biggest principalities, so:
Volhynian culture - historical lands of the Volhynian Duchy
Halychan - the Duchy of Halych
Polesian - the Duchy of Turov-Pinsk
Polatskian - the Duchy of Polatsk
Smolenskian - the Duchy of Smolensk
Severian - the Duchy of Chernihiv
Ruthenian - the Duchies of Kyiv and Pereyaslav
Novgorodian - the Republic of Novgorod
Muscovite - the Duchies of Vladimir-Suzdal and Moscow
The core territory of the Duchy of Chernihiv is called Severia till now and it is the same cultural ethnographic region of Ukraine as Volhynia or Galicia. And the people that lived there at that time were called '
Sevruyks', they were kind of unique because they got used to live under much more direct mongolian rule than other southern Ruthenians. So it fits perfectly into the game logic.