I think the uniqueness of English can kind of be quantified by its rate of divergence. That is what the author sort of gets to by saying that there are no mutua

I think the uniqueness of English can kind of be quantified by its rate of divergence. That is what the author sort of gets to by saying that there are no mutually intelligible languages. I think that if we could quantify that rate (by time) of divergence, we may see that English is pretty far on one end of the scale. Consider that the fall of Rome and the Anglo-Saxon settlement of Britain occurred at roughly the same time (~500 CE). Then compare how far English has diverged from other Low German languages, to how far Spanish and French have diverged (also consider that those languages were already on their way diverging from Latin with a head start on English). English diverged a lot faster. As to why English diverged faster, I don't think the author gives particularly good reasons. "Inter-linguistic" migration/settlement waves are not unique to Britain. Most of his arguments for English can be applied to French. The Celtic Gauls were "invaded" by Latin speakers, and to some extent started adopting it as a second language. Then Germanic Franks (and others) started flowing into and settling into parts of what is today France, again adopting the language. Then waves of Norse speakers settled into Northern France and that had an effect on the language. But through all of that, French still isn't as divergent from Latin as English is from Old Low German. What is the difference? I think the actual answer is probably largely due to geography/topography. English diverged faster because it was more isolated, and left more to its own devices without as much contact with its divergent siblings. I think the extent of contact with siblings is the force that keeps languages from diverging. It's probably the same reason that Japanese is so divergent (from whatever it is that it diverged from). Similarly, it's why small "isolate" groups tend to cluster in mountainous regions. Basque/Georgian/Korean would probably not exist today if they had originally developed on some flat plains in the middle of Eurasia. I went a bit long on this topic, but there was an argument for the author to make. I kind of wish he had made it.