Interesting to see Marin being the unexpressive one in a scene, but she's got a lot to think about and her dad was there to pick up the slack.
And naturally Wakana fails (or seems to fail) to pick up what her dad was putting down. But if you want to grind your teeth over Wakana "being dense", consider how high-context Japanese culture is and how ambiguous statements such as dad's leave a lot of room for misinterpretation, particularly when you consider Wakana's low self-esteem. Rather than simply missing what seems to the reader to be obvious subtext, if his distorted self-appraisal of their relationship is that his only real contribution is as her dietician then that's the subtext he's going to pick up on.