Orthodoxy says that English was influenced not just by Anglo-Saxon but also by the Norse/Danish and Scandinavian invasions.
Harper admits his is a work of assertion. He fails to describe the 'orthodox' positins accurately and sets up strawmen. He doesn't even bother to tackle the real linguistic arguments.
The experience of England after Roman rule collapsed was unique, and Harper glosses over this.
A lot of 'musts', strawmen and assertions and ignoring the work that has actually been done on the history of the English language may convince those that know nothing about the subjects Harper covers, but those that do know anything about them will realise how fragile his arguments are.