Basically, it's not that people born after 1980 are narcissists, it's that young people are narcissists, and they get over themselves as they get older. It's like doing a study of toddlers and declaring those born since 2010 are Generation Sociopath: Kids These Days Will Pull Your Hair, Pee On Walls, Throw Full Bowls of Cereal Without Even Thinking of the Consequences. Further, they write:

In turn, when older people are told that younger people are getting increasingly narcissistic, they may be prone to agree because they confuse the claim for generational change with the fact that younger people are simply more narcissistic than they are. The confusion leads to an increased likelihood that older individuals will agree with the Generation Me argument despite its lack of empirical support.

Hahaha, you doddering old confused fools! Generation Abe Simpson!