This was a Twitch bug that has now been fixed, thankfully.
There was no code to show in this instance, other than the declaration of the onAuthorized callback. Not tremendously interesting:
var twitch = window.Twitch.ext;
...
twitch.onAuthorized(function(auth) {
The error occurred when the broadcaster was trying to use their own extension. I did not get the full details on why it was not happening for my extension, but it is fixed now. I wonder if something similar happened for the four month old thread. I wondered why it was sitting alone and ignored for four months in a corner of the internet. 
The only thing unique to my extension is that I don’t use a framework.
Thanks for the tips, @BarryCarlyon!