I would dispute your definition of "justification". I would also draw a distinction between "definition of knowledge" and "knowing that you know". I.e., that you know that the viewer doesn't know, even though he thinks he knows, is itself grounded in justification (here, the knowledge that you have put up a fake background).

I could just as easily construct a problem in which I quietly turn off your background, which would mean your Zoom partner does possess knowledge while you do not, even though now it is you who thinks he does.