There is no previous mention of any plan!
It's like in The Black Camel where Charlie says something about a conversation he had with the Australian couple that morning. Any such scene must have been deleted since we didn't see it.
That is a good comparison. The viewer is expected to connect the dots, evidently. It may, however, have been a case of a continuity error in "The Black Camel."
In "Charlie Chan in Egypt," the audience was as much in the dark about Charlie Chan's plan as the killer. It does add to the scene's suspence, and if anyone has not guessed the identity of the killer, it provides an immediate revelation.