Forza Motorsport/Horizon getting Xbox One versions is something that makes complete sense to me honestly, given what we know about Turn10 and Playground.
They have full control of their engine, and they also both have successful games in their respective series that ran on the Xbox One as well, so I have to imagine that even with a full next gen update to their engine and all that entails (rendering tech, texture streaming, ray tracing etc) I have to imagine that they'll be able to backport the non-graphical elements (like simulation elements and some physics stuff) to be able to make a cross gen game that will be able to emulate the next gen versions on last gen without really holding back the last gen versions. Not even in a Forza Horizon 2 kind of manner (they'll likely have to compromise in Horizon for world design to accommodate the Xbone but I digress) but in an actual cohesive way.
Like the graphics will likely be the same or worse than their previous Xbox One counter parts on the Xbox One and not at all really held back on Series X, and gameplay wise I'm pretty sure they'll be able to pull it all off too, probably by offloading some stuff to the GPU or something
Also, if both Horizon and Motorsport are cross gen, that also pretty much guarantees 120fps modes as well