Russia claims to be descendent of many things, from vikings to Byzantine Empire, from slavic tribes to mongols. Russia is a mix of many things, where Kiev and Kievan Rus is just one of the main starting points. Not to mention that Kiev became the capital only ofter Oleg captured it after leaving Novgorod. Not to mention that any fights between Moscow and whatever entity you are talking about have happened centuries later.
And even if we are talking about geography only: https://en.wikipedia.org/wiki/Kievan_Rus%27#/media/File:Prin...
here is the map of the Kievan Rus (1054-1132). Lands of the modern Ukraine and Belarus take less than a half of the territories as can be clearly seen. Even considering that this is a 'flat' map.
And you can't really have any talk about Russian state over here without mentioning that 'Russia is still a Byzantine Empire descendant'. Even newspapers usualy mention that: https://www.gazeta.ru/comments/2016/05/30_e_8271917.shtml (in russian, obviously)
Anyway, this topic has lots of material if one is interested.