What are the formative foreign policy experiences of "Millennials?" Probably none. Why?
Most Americans of all ages do not pay too much attention to foreign policy, outside a small community of policy wonks. It is rather laughable to assume that a large mass of people would consciously describe a foreign policy event as a significant influence on their worldview. The end of a first romantic relationship is probably a great deal more important to most people than the equally messy breakup of the Soviet Union.
If foreign policy matters, it is as a pure extension of partisan domestic politics or narrow controversial issues. Relations with Latin America as a whole, for example ( if they matter to most Americans at all) are seen through the prism of illegal immigration from Mexico. Events like 9/11 or Iraq tend to confirm or challenge partisan or cultural views most Americans already have.
I think that commentators take the Cold War--which tended to buck the general trend in some ways (e.g. apathy was impossible in the face of nuclear annihilation, although partisan politics was still the same as it always was) and then imagine that most people today still see the world the same way.