Iron Age study targets British DNA mystery

I'd love to understand how the DNA modelling from archeological remains found, curated before we knew how to do PCR avoid contamination issues.

Basically, how do they know the level of contamination in these huge held stocks of pots and fragments?

For future finds, I can totally get behind a protocol to collect from a layer with huge amounts of avoid contamination behaviour. But did they do that enough during the golden age of digging?