I think the point must have been made about the people of Brittany, not Normandy. Norman dialect now is just a form of French. Originally, in the Middle Ages, Normandy was settled by Norsemen, but they quickly adopted French as their language. By the time they conquered England in 1066, I don't think there were any traces left of their original Scandinavian language.
It is astounding that you can be so cock sure the Norsemen totally lost their lingustic heritage IN ONLY THREE GENERATIONS or less, but everyone else you promote doesn't lose theirs in THOUSANDS OF YEARS! Don't you consider this ironic?