Note: this is just further info, I'm not disagreeing with the text.

All words are different in an idiolectal level. That is: even if Alice and Bob speak the same variety, they associate every single word with a bundle slightly different concepts and use slightly different pronunciations. And if they speak different albeit related varieties, the cognates might've drifted in pronunciation and meaning that they're barely recognisable.

And when it comes to borrowings the words are often different in a morphemic level. "Chocolate" mentioned in the text is a good example - English takes the word as a single unit and call it a day, and yet in Classical Nahuatl you'd probably analyse it as four morphemes (choco-l-ā-tl, roughly "fermented liquid"), and it referred to the beverage, not the solid stuff. It's hard to claim that it's the same meaning.