They were able to extract DNA from a single premolar tooth taken from the mandible. The tooth’s cementum and enamel were removed, the tooth was ground into powder, and the sample was zapped with UV radiation to ensure it wasn’t contaminated. To determine ancestry, the team compared the Well-man’s DNA to a database of more than 6,000 modern-day Norwegians.
I also found that a bit unexpected. But also, they may be banking on the idea that before science was a thing, you might not be certain that regular decay would poison the water and maybe you toss in the guy with plague to ensure you get the desired effect (not that you wouldn’t throw in an non-diseased person if you lack a plague guy).