Well it's hard to explain really, and my version will be a bit weak since I've never read Watership Down.
An illusion is a concrete image, there's no question what a person is supposed to be seeing and the emotion that image is supposed to invoke.
An Allusion accomplishes the same emotion by using a different image. Arthur and Fiver's relationship alludes to the relationship between Hazel and Fiver. The same feeling is being invoked in a totally different enviroment and situation.