From that quote, I would assume the author's argument is "Math contains many unapplied topics, whereas CS contains many applied topics. Therefore CS is more us

From that quote, I would assume the author's argument is "Math contains many unapplied topics, whereas CS contains many applied topics. Therefore CS is more useful for outside-domain work." I would suggest looking for applications of say, linear algebra or group theory. I don't know enough CS to come up with an equivalently unapplicable topic to match elliptic curves.