「リベンジ」 doesn't really directly translate as "revenge", it's a loanword that's used in a lighter manner to describe trying again at something after failing, without the implied connotations of retaliation, personal vendetta or exacting vengeance on someone for hurting you as it does in English. If they had used the word 復習 then that would've been translated as "revenge", although that would be way too strong to describe the relationship between Orihara and Nagatoro based on what we've read so far.