What clicked for me abruptly when reading this lesson is that there is a relatively good translation that comes to mind: "whatever". Now, my English grammar isn't academic level, but the original meanings of both "n'importe quoi" and "whatever" would appear to to suggest "anything".
Example: "take anything you want" / "take whatever you want"
The similarity is astonishing when you consider the other use of "whatever", namely, "used to express astonishment or perplexity"
Example: "whatever do you mean by that", but also "Whatever!"
I'd be interested in hearing whether this brings clarity on the popular meaning of "n'importe quoi" in French, as it seems to parallel English so closely in its deviation from tradition.