This is a fairly stupid conclusion. To believe it, you would have to think that the 200,000 Anglo-Saxon males brought their women with them to ravish and pillage fair England. I doubt it.
They came and conquered a portion of central England, then took the Celtic maidens as the spoils of their conquest - breeding lustily with favorable laws, etc. The result is the 50% Germanic Y-DNA chromosome, which is passed only from father to son. It doesn't matter whether the mother was Celtic or German origin. Surely, she was Celtic in the main.
Also, wouldn't the Normans have carried Germanic DNA?