And I think @ljharb is referring to the fact that in ES3 engines (IE <= 8) all of the global properties can be redefined or shadowed (ie, `var undefined = 32;` somewhere in the long sequence of JavaScript you've concatenated together).  http://designpepper.com/blog/drips/redefining-undefined talks about this a bit.
It's not an issue for ES5 engines.  I don't know how much we care about ES3 compatibility.  I don't have a strong feeling either way; like I said, I was just cut-and-pasting.