Parent meant problems that were in the past of interest to research mathematicians. Someone(s) published the first papers exhibiting Catalan numbers underlying various counting problems. Obviously we don't expect elementary students to be at the forefront of modern cutting edge research.
If so, that's a much weaker and not terribly interesting claim. The concrete example of counting with Catalan numbers is more compelling; it would be very challenging for children who lack experience with recursive definitions and inductive proofs. Systematic enumeration, albeit elementary, is distinctly modern.
