This might be a shock to you, but words can have different meanings based on context. Even in your own link, the reference to baby boomers was the last entry.

When used as an insult it has a different meaning from when it's used as a term for an age range.

When you call someone a bastard as an insult, you're not calling them a child born out of wedlock. You're calling them the negative connotations of the word.

When you call someone an asshole, you're not literally calling them a rectum. You're saying they are like the negative associations with the word.

When you call someone a boomer as an insult, you're not literally saying they were born in the baby boomer year range. You're calling them old and out of touch, like a baby boomer.

The insult is an analogy, not a description.