I said boolean logic because the paper mention fifth graders, which are unlikely to be at ease with complex boolean formulae. But there are number of more complex concepts that would also work: OOP, maps or reduce, mutexes and so on.

To answer your question, in my experience (I taught programming 101 number of times) many people are indeed confused when they need to reason about complex if statements. Typically, they make redundant tests, and it takes a little while before they understand why it is not needed.