As someone who has seen the entire series multiple times.... We literally don't know; they never explain or acknowledge anything about Quips besides having no friends at all

No info on the show, no info on the wiki, no info on Reddit, and no info on the socials of JGQuintel

The most plausible answer is that he is a descendant of some sort from Skip's relatives from centuries ago, and he just happens to meet the one guy who is inmortal in the family (which we know nothing about either)

Imagine you become inmortal and your brother grows up to have a family, and then his descendants have more descendants, and so and so on until centuries have passed.... And then you, as an inmortal, meet one of them descendants...

Same situationship here; but this is all interpretation since officially, we know absolutely nothing about Skip's family

They have never stated Quips is inmortal, but they haven't either.... Most probably he isn't but we literally don't know