Actually, certain Diancie can be traded online. I was able to, for instance, trade the July 6 movie pre-release Adamant Jirachi (31/31/xx/31/xx/xx) with no difficilty, and I even cloned and swapped clones with someone else (one was levelled up to Lv.100, the other as obtained at Lv.50). Only Japanese Diancie are legit. ATM.
Unless the post meant a Shiny Jirachi. Yeah, that's a hack.