This particular некого is unrelated to кто. It's from некий, which is basically a more bookish, slightly archaic какой-то.
Properly, the genitive should be некоего, but it's one of those quirks of inflection that a lot of people forget or never learn.
Here, the choice of неко(е)го over какого-то adds an undertone of scepticism or suspicion (a "some king" that stops just short of saying "some supposed king") whereas with какого-то, it would've been more outright dismissive ("some king... like those are ever any good").
EDIT: Ouch. I completely forgot — despite your translation — that некий can also have a different shade of meaning, close to кое-какой. So without context, it's hard to tell whether the speaker means "than some king" or, indeed, "than a certain king [we all know]".