What I don't understand is why you think any unaudited code is secure. There is no mechanism by which code from an unknown source becomes trustworthy without someone reading it, or being able to read it.
Nowhere am I suggesting that understanding the javascript source is quick or easy, but it is in fact possible, which is the difference between encrypting client-to-server vs. client-to-client. Source code you can view is not automatically trustworthy. Of course not. But it is easier to trust than code you will never be allowed to see. Can we at least agree on that much?
As far as trustworthiness, I would say: local code > remote code run on the client > remote code run on the server > closed source. This makes (a theoretically fatal-bug-free) bonchat-like system worse than PGP, but better than Facebook IM.
It sounds like your real complaint is with the "marketing", such as it is. Perhaps I should do a better job labeling it as experimental, or a proof of concept? These would be actual concrete criticisms.
Folks complaining of XSS vulnerabilities seem to be under the mistaken impression that it somehow secures you against the people you're talking to, which is not the problem it's trying to solve. Unescaped HTML chat is vulnerable to XSS, which is why you shouldn't exchange raw HTML with random strangers. You wouldn't open an HTML attachment from a stranger, but you'd open one from your good web developer friend.
(A very fair complaint is that, although XSS is irrelevant to the intended use, I should've anticipated that people would want to test/demo the basic chat features by broadly distributing passwords. This turns it into, essentially, a plain text chat with two URLs you have to enter. Which is useless, but perhaps amusing, so I've added default HTML escaping.)