It doesn't matter that it theoretically all happen in the browser. You can serve different versions to different IPs etc. Every heuristic in me would be screaming don't use that if I would have a need for such tool.
(Sorry for commenting on you twice in this thread--I promise I'm not trying to follow you around. I'm all about dissecting the quality of a system or toolchain! A false sense of security can be more dangerous than naïveté! Caution and skepticism are often our only protection! Trusting random websites is Not a Good Idea! But, security tools average people can't use are also meaningless here.)