O_o
Well, I am going to give you the benefit of the doubt and assume that somebody told you it was in the book. I am also going to assume that you don't have a copy of the book to look it up yourself.
No, I checked the index and it doesn't make any mention of the special rule "Elite Army." In the FAQ when the rule is explained, it specifically states "High Elves" so one would assume that it is an amended new army wide special rule for high elves.
Thank you for saying please, and you are welcome.