The part where Gen-X is younger, maybe?

It's perplexing, to put it generously, but it doesn't throw the semantics of the entire sentence into question.

For all we know the document is from two decades ago.

Two decades ago makes the GenZ reference confusing, as the very oldest of them by the most generous definition would be only 9 years old.

With a span across 50 years, that range from Gen X to Gen Z is just awkward to place as "young buyers of Monster" at any point in time.