It doesn't state the resolution of the 60fps mode (Series X only). So likely 1080p but they'll be trying to push it to 1440p. I'm assuming its built on the FH4 engine/tech and until Turn 10 get to grips with the new engine we won't see Ray Tracing or 4K60 on next gen consoles in a Forza game.
edit : 60fps on Series S too impressive wee box that. In that case I suggest they aren't stating 60fps resolutions because they know the internet will jump all over that shit in an attempt to smear the game.