forza 5 anti-aliasing ???

Does that help with the anti-aliasing or no?

It's the graphics that cause that. Have a read of this, helps explain it somewhat and how games try to prevent it.

www.overclock.net/.../anti-aliasing-the-basics

Digital Foundry actually being positive about an Xbox One game??? Surely not?

Digital Foundry: Hands-on with the Forza Horizon 2 demo

"Further inspection of the demo shows that Forza Horizon 2 delivers its visual punch with a crisp native 1080p resolution, as promised by the developers earlier in the year. The anti-aliasing solution is also given a boost over that of Forza 5 via the use
of 4x MSAA (multi-sampling anti-aliasing) - a bandwidth-heavy choice that we rarely see in use on console."

"Impressively, despite the demanding open-world set-up and an avalanche of advanced rendering effects,
Forza Horizon 2 holds a solid 30fps throughout the demo without ever dropping a single frame during gameplay, no matter how carelessly we ploughed through the scenery and collided with other vehicles - driving feels smooth and responsive in
a variety of weather and road conditions."

For pp with Full HD 1080p HDTV's, a 720p native game would have to be "stretched/upscaled" to 1080p. This means you would probably can expect "jaggies" unless Xbox One has some sort of anti-aliasing hardware which the Xbox 360 has 4x anti-aliasing in it's
EDRAM is it. That's the only downside, but if it has really good textures/frame rate to compensate, then that's great. While 720p is HD, it's not a high enough resolution to run graphics without the need for anti-aliasing, but 1080p doesn't really need anti-aliasing,
especially for Full HDTV's.

jagged edges has more to do with lack of anti aliasing, not your hdmi's fault. The clearer the image, the more visible aliasing is.