It would seem that the only person allows to window.Twitch.ext.send with broadcast is in fact the streamer that the page is installed upon.
Not sure if a bug, or a piece of information missing from the documentation.
I checked the response from my “viewer trying to .send on the topic broadcast” the result response was “Permission denied to use target” Viewers cannot “broadcast”
Only the “broadcaster” can “broadcast”
After doing some tests. Seems that a viewer cannot use window.Twitch.ext.send at all. Only the broadcaster can.