Unfournantly, to call the stereo image an illusion is a bit problematic as some people seem to associate the word illusion with "magic" tricks.
On one hand, stereo reproduction of sound is a trick on our hearing with two speakers playing the same recorded sound object which will then appear to be coming from points somewhere between those two speakers. But on the other hand, if the recording was indeed capturing a real sound event, all the captured cues are as real as the distance and placement of the microphones used for the recording, there's no illusion used for that capturing part.
I'm pretty sure mono is an illusion: there aren't any tiny musicians in that box.