I must admit I don’t love the writings of Jean Twenge. One of her first popular works was basically a book calling millennials ‘uniquely narcissistic’, which has been widely challenged:

https://link.springer.com/chapter/10.1007/978-3-319-92171-6_20 https://journals.sagepub.com/doi/abs/10.1111/j.1467-9280.2008.02065.x?journalCode=pssa https://journals.sagepub.com/doi/abs/10.1177/1745691609357019?journalCode=ppsa

She seems to write in the vein of ‘scientific sounding’ pop culture simplifications of how things work. Simple paradigms are attractive to people. We like to categorize, in fact we’ve evolved the tendency to categorize everything because our brains cannot function otherwise. So, she categorizes the generations into basic caricatures and tells everyone that this is all you need to know, and that kind of feels good.

The reality that narcissism may increase with age or that narcissism may be the same in her own generation is unpalatable to her simple generation categories, so thats not discussed because it creates more grey area and less black and white.