I've seen this happen a number of times. Kobolds are much weaker than the other races, but they can engage in site claiming/reclaiming like any other civilization. I'm not sure if the Kobolds killed the previous owners (who were likely weakened by a rampage or war with a more powerful foe) or if they claimed the site after everyone else died in it.
I've seen this, i even mentioned it to toady in a FotF post once. Kobolds jumped into a human halmet (i've seen towns/dark towers etc) and then stole the job roles and occupation of people living inside. I swung around batting away every kobold i could chase as part of a quest where apparently the abandoned town was infested with miscreants or some fluff like that but couldn't complete it. Just a bunch of wide eyed dopey civilians like threshers etc, not even any guards i could see (so still could be kobold role types, i don't know)
Personally i think its to do with faulty [CONQUERED_SITE] nobles or lack of them, where races who overwhelmingly conquer others without the relevant noble just replace nobles and eventually end up with a site leader transferring over control. (for example, goblins who become barons in conquered/refugee dwarf civs can't manually replace so put goblin barons in charge who conveniently are site ruling land owners aligned with the goblin civ rather than a flat out generic [CONQUERED_SITE] noble)
If the elves all get killed in a war, there are still depopulated sites still in elven entity control while the race still lives (once i had a evil civ human inherit the human civilization by founding a new town in control of the goblin entity when all the human sites were lost, probably a cousin to the prior law-giver taking the title)
Depopulated sites because civs prefer to pillage & murder instead of raze sites they can't use is a problem, and skulking civs launch "attacks" (which could be how they are getting in) on civilisations who have a lot of goods stolen. Kobold walks up to empty site with a very weak attack force = bam, easy takeover.
Alteast once also i think i've seen goblins prompt to declare war on a kobold held site, though its not tracked or detailed in legends so we'd never know (it was lower case, not flashy yellow)