Baton Pass Xatu is only available in Gen III, and learns it from its previous evolution Natu:
Natu special moves
Natu learns it in a side game, Pokemon XD: Gale of Darkness. So in order to get a Baton Pass Natu in Gen V, you need to purify Natu, transfer it from Pokemon XD to Emerald/ LeafGreen/ FireRed, then migrate it via Pal Park into Gen IV, then trade it to Gen V.
Technically, if you had a male Natu/ Xatu with Baton Pass and a female Natu/ Xatu with Magic Bounce, it would be possible to breed a Magic Bounce Natu with Baton Pass.
I guess it was either an oversight on Showdown's part, or they didn't think it was practical since nearly no one would have a Magic Bounce Baton Pass Xatu in Gen V.
Give it this set instead: