For gen X and older, computers were more niche. They were more difficult to use and mostly reserved to enthusiasts. For gen Z and younger, computers were always just there, and they'd become a lot simpler, a lot more plug n play, and resources to fix them became cheaper and more accessible. Millennials were in just the right environment where computer use became mainstream, but computer software was less developed and user friendly and they frequently had to learn to fix problems themselves.
Every individual will clearly have their own unique experience and not everyone will fall into these buckets, but it's these factors that lead to millennials likely having higher tech skills than average compared to those older and younger than them.
That's it. It's not ageism. It is absolutely generalizing, but mostly, it's social commentary in the form of a joke.
