i fail to see how codependancy is inherently a bad thing. sometimes it IS healthy.
in this case i see it as healthy they both complete what the other lacks. nana lacks a internal battery and she can't use magic without one. the princes has an overflowing internal battery but no outlet for it all to go resulting in random outbursts of energy.
together though nana acts as the outlet for the princesses overflowing energy and in turn uses the excess energy to use magic. and its not like they can't live without eachother. the princess has been living before nana was summoned and already had people who could manage her condition and its not like nana can't live without magic.
the unhealthy aspects of codependancy is when negative effects occure when the two people are apart. and obviously that isn't the case here.
What you say is true, but you may be missing the conflict here. Nana has always struggled with self-esteem, and despite suddenly becoming one of the most important people in this world, she sees her own value and utility entirely dependent on the Princess. The Princess is worried that Nana, the saint who came to possess an incredible ability with no training, might leave her behind. So she feels relieved to learn that she is the only magic source that Nana can use to cast spells.
This conflict emerges from the girls' psychology, not their physiological or magical requirements to function. They're matched so perfectly that even their psychological hangups feed on each other. And I'm glad, because I was a little worried that this story was going to be too wholesome. I don't think the people calling this couple "codependent" mean that they are doomed to a bad relationship. They are just pointing out an obstacle that the girls will have to deal with at some point.