You're not misinterpreting. Norsemen, the Vikings (or Varangians as they were called east of the Baltic Sea) were of course Germanic. And the prevailing view is that they founded the medieval state that we now refer to as "the Kievan Rus". The Vikings, however, were only the ruling elite. The vast majority of the population was East Slavic. Over time, the Vikings adopted the language and customs of the people they ruled, essentially assimilating themselves into the Slavic population.

This is roughly analogous to the Germanic tribes invading the Western Roman empire. Initially the Germanic-speaking ruling class and the Latin-speaking populace were distinct. Over time, the Germanic elite adopted the language and customs of the local population.