length of an array, and elements of arguments object outside strict mode, are not actually accessor properties, but rather proxied data properties

[lewisje]

TL;DR: It might be better to say they act like getters and setters, with a footnote explaining they're even more exotic than that.

The length property of an array, and the elements of the arguments object, are actually data properties:

Object.getOwnPropertyDescriptor([], 'length');
//=> Object {value: 0, writable: true, enumerable: false, configurable: false}
(function () {return Object.getOwnPropertyDescriptor(arguments, 0);})(undefined); // passing in no arguments returns just `undefined`
//=> Object {value: undefined, writable: true, enumerable: true, configurable: true}

The behavior of both of these things can be approximated by an object wrapped in an ES6 Proxy but maybe talking about that would be too much, considering most of this guide is not written for ES6; also, to my understanding, the link outside strict mode between the arguments object and named arguments cannot be so approximated, because the scope object that the named arguments are properties of cannot be accessed via script.