That's so weird. I bought the standard edition (definitely this version) digitally for cheap just after BC, and it did come with Undead Nightmare.
Yeah, for whatever reason it came with the regular game and no Undead Nightmare. At the time I didn't really mind because I had the wrong idea about Undead Nightmare. I bought it pretty soon after it was made available on digital.
Still well worth the $20 I got for the incredible game, I just wish I would've at least got the GOTY edition for the $5-10 more it would've cost.
I did see Undead Nightmare for $5 during the big Xbox sale from November, I just didn't bite because my backlog was/is huge.