It was a huge amount of work that paid extraordinarily well. One could argue that Y2K was a kind-of Keynesian program for software developers, spawning an entire generation of 90s upper-income schlubs and mini-millionaire bosses.

Everyone from Marc Andressen to Mark Cuban owe their personal fortunes to the enormous infusion of cash into the IT sector created by the Y2K business boom. Incidentally, the bottom fell out of the tech market in... 2001.

Sure, but millenials didn't do that work. Most of us were like 10 when Y2K happened

Yes, but it's being mentioned alongside things that widely and tangibly affected the (American) populace. Y2K did not have many actual effects for the average person, just a couple of knock-on effects created more from people's fear of Y2K.

But the people putting in that effort weren't millennials, it was their parents.