For most of human history, and even today, much of our individual identity is heavily tied to our familial identity.

Saying that someone’s mother is especially promiscuous is basically saying that you can’t trust any claims about what their true family tree is, or that it is that way on purpose.

The reason the insulter would use themselves as an example is because they clearly don’t have any romantic interest in her.

It’s less about it being an accomplishment for the insulter, and more about it meaning nothing at all to them. That you may end up with a half-sibling as a consequence of nothing truly significant.

It’s as if to say that your family’s constintuency is so carelessly crafted that the entire reason you exist at all may be that someone offered your mom an Oreo for a handjob and she counter-offered with sex for the whole sleeve.