The author doesn't properly understand the meaning of the word "rivalry."
To "rival" means to "compete" or "vie" with - usually in a sporting or at least friendly context.
Regards,
Talking about oppression and slavery, the Blood tax (devisme) required every fifth (5th) Christian boy to be given to sultan into slavery! To be converted and indoctrinated. Even now the unruly children on Central and East Europe are scared - If you do not behave, the Sultan will come and take you away! I remember my grandma was telling me that when I was young.
https://en.wikipedia.org/wiki/Slavery_in_the_Ottoman_Empire#Ottoman_slavery_in_Central_and_Eastern_Europe