If I have the definition of this correctly interpreted, I "see" an "illusion" as something "visual". An oxymoron perhaps. Whereas, a delusion is something of a mental nature in terms of "belief". Perhaps Freud didn't consult his dictionary?
eg: A paranoid person is deluded.
A person in the desert seeing a mirage sees an illusion.
Very early in the morning here for me so perhaps I'm also deluded.

"Adversity is the whetstone of creativity"