I think that is not the case... they used the Y chromosome. Not being a connoisseur, I have copied the survey's info for you from the Blood Of The Vikings site:

"The UCL genetics survey set out to discover if any genetic traces of the Vikings remained in the British Isles - and what this might reveal about the Viking Age. We hoped to find out where Vikings settled and roughly how significant those settlements were. DNA samples were taken from men at a number of sites. In the main, small towns were chosen and the men tested were required to be able to trace their male line back two generations in the same rural area - within 20 miles of the town chosen. The aim was to reduce the effects of later population movements, assuming that in between the Norman invasion of 1066 and the 20th century movement would have been limited.

The tests looked at the Y chromosome, which is only carried by men. This chromosome is particularly useful for population genetics studies as it is passed directly from father to son with virtually no alteration. Other chromosomes exist in pairs, one member of which is passed on from the mother and the other from the father. But because women do not carry a Y chromosome, geneticists can always be sure that this part of a man's DNA has come from his father, and from his grandfather before him. This chromosome allows geneticists to begin to unravel the male ancestry of the British Isles."