She said "very little" Norman genes, not none at all. Given that the normal story of the Norman invasion is not about mass invasion, but the replacement of one ruling class with another, so relatively few actual people, this would not seem to be surprising.
She also said that about Sub-Saharan African genes. Given that those were probably not a majority of the Roman forces, and the Romans didn't exactly mass-migrate to Britain either, this is also not surprising.
