I notice that he says Middle English was a product of the merger of Old English and the Scandinavian language spoken in England. I always understood that Norman French was the greatest factor in the shift from Old to Middle English.
Norman French added a lot of vocabulary, but the language structure was not much affected. German nouns and Latin/French verbs are totally different from English, and much harder to learn. Observed by one who flunked German in high school, and almost flunked Spanish before finally catching on. Now speak decent Spanish and have studied some Italian, French and Portuguese.