Isn't a boomer somebody who was born during the baby boom? What's the boomer phase of life?

Boomer seems like it’s used as a random negative descriptor rather than what it actually is as a generational label.

If you want proof it's a thing, see https://en.wikipedia.org/wiki/OK_boomer. That's a pretty wretched source tho. It's written by and for the kind of people who think citing thinkpieces in The Guardian can have any relation to how slang is actually used in real life.