The Guardian of Forever literally told Burnham and Georgiou that Saru would lead the rebellion and go on to save others because Georgiou saved his life. It's only natural for Burnham to make that assumption based on the information she was given. Also given the fact that Saru, a kelpien, is a main character, of course it will be him.
This season has a trend of using inconsequential lines of dialogue to foreshadow plot points. It's possible Saru is going to find out that he led a rebellion in the mirror universe and it's going to tie in with the rest of his story arc this season