Well, the author intends to give us advice on the the general purpose CS stuff which all the CS students should have a practical knowledge in.

I think its taken for granted that while graduating, we would be doing the more demanding things such as: -designing our own programming language. -developing a minimal OS dedicated for some specific task. -develop an new alogorithm or drastically improve upon an existing one. -etc...

While doing the things mentioned in the author's list, you may surely implement a few of the crypting concepts which you have learned during your graduation. No one is stopping you to do that.