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, rather just used it for the specific Rus tribe which the author then in turn generalizes upon all Scandinavian or even Germanic tribes.