'use strict'; should be in the top of the scope you control. A global 'use strict'; would place the global scope into strict mode, so all your imported modules would be under strict mode too, and then third party code would break and you don't know why. IIRC jshint will pick this up.

To be honest, the whole guide seems like a shorter, less comprehensive version of the JS community's own Idiomatic JS https://github.com/rwaldron/idiomatic.js/