Take everything you read from German authors on such topics with grain of salt. Not often will they take also such Finnish or Slavic loanwords and later portrait them as their own (let's not forget the Germanization process of neighboring nations of 20th century by Germanic and many Germanic authors, philosophers and historians) etc.

Rurik, founder of the Rus himself by his genes belonged to the Finnic N haplogroup. Not Germanic I nor Slavic R1a. That fact alone speaks enough imo.

Personally i never heard Slavs use such word for Scandinavians, thought an input of Baltic Slavs as Poles, Belarussians or Russian would be valuable here…i highly doubt they use (or used anything similar) for Scandinavians as well.

Quite true, I read something about that topic yesterday aswell and I was disgusted about how the book claimed mostly everything about early Kievan Rus', knez Vladimir, Igor, Novgorod and more to be based purely on Swedish Vikings.

Although it is kind of generally known that those Swedish or whatever Vikings should have been called Rusi by the Slavs there. What about the name Varjagi? I sometimes come across it but never really know whom they refer to with it. Is it used for the same guys as Rusi?