Odd that people are taken reading so much into what Unohana was saying never did I get the impression that Kubo was trying to imply that Zaraki as a kid was stronger than Unohana at that particular moment he was fighting at his current limit it was a first time that he was able to let loose and she did not, as shown with her shikai not even being released,I believe that there is an ulterior meaning when she said "even I was not a match for you" leaning more to his thirst for battle. After experiencing battle with Unohana that sense of euphoria he felt he wanted something stronger but she could not give it to him.As he got older that feeling eluded him and hence the shackling only when he reaches his full potential and fight someone who he can let loose again will he surpass that sense of pleasure he had with Unohana in battle.
EDIT:She clearly considers him a worthy successor and saw the potential he had as a kid ,why would she say "Finally surpass me so you can obtain new heights" if he was stronger than her in the past.
EDIT:She clearly considers him a worthy successor and saw the potential he had as a kid ,why would she say "Finally surpass me so you can obtain new heights" if he was stronger than her in the past.