I see what you mean, but it just boils down to how you interpret the problem. Specifically, if you focus on the actual forwarding paths instead of the contact lists, the tree structure that enables the linear solution becomes more clear.

As I’m guessing was intended, this problem is particularly well-suited for engineering interviews because you can start with an easy example (the DAG one could derive in this specific interview) then ask the applicant how the solution would change given the additional constraints you’ve described.