Well, now you can, but it doesn't seem like you are supposed to. As spotted by NeoGAF, you can get access to the game on Xbox One, but only if you own it digitally. It is then explained:
OP, put that you have to follow "rockohoward", view his profile, view what he follows, and then click RDR to get it.
Undead Nightmare, the massive expansion for the game, doesn't seem to work sadly.
My guess is that this was a mistake of some kind, but I'd assume this means that the compatibility was being worked on and has now leaked. I'd expect to see it in the near future.