You are asserting a made-up argument. Having an active public project behind a logo is not a necessary condition for not re-purposing the logo for another project. This particular concept just happens to be taken.
Every good logo is trivial in retrospect, it's finding a strong concept that's a bitch. I looked at /gd/ thread and virtually all of it revolves around shaping a keyhole this way or that way. Then suddenly someone says - "Here, done". Turning an empty space into a chat bubble requires a step up, it's an altogether stronger and more complex concept. It is possible that another person came up with it independently, but also consider that with all the variety of secure messaging apps no one had thought of it before 2012. It's not an easy concept to stumble upon.
(edit) This is getting meta. It is plenty obvious that Tox fellas think they are in the clear and it's OK to recycle an existing logo. I think that it's not. That's hard to reconcile.
