The Kent kingdom was destroyed within half a month.
The Seven Kingdoms Era of Great Britain is still in its infancy, essentially a loose tribal informal alliance. In fact, Duncan didn't just destroy one country; he also incidentally wiped out what would become the future West Saxon kingdom.
In this era of England, just a few thousand people would dare call themselves a kingdom, with territories possibly the size of just one or two counties.
East Anglia is one of the future formal Seven Kingdoms, but its territory only includes Norfolk and Suffolk, which is smaller than a large prefecture-level city. Duncan was caught off guard when, a few days later, Kvito gleefully reported that he had destroyed a small kingdom established by the Anglo people.
Very well.
This act of annihilation is enough to elevate him to nobility, considering he was one of the earliest commanders to follow Duncan. Among the exiles, some should be ennobled.