We don't really know how the R9 280 and lower GPUs will perform in this game until it's out. So saying the XBox One S runs RDR2 better than a 3TF GCN PC GPU is just guess work based on the system requirements until it has been tested.
We don't even know what the minimum system requirements are targeting in terms of graphical settings as they're quite vague to say the least.
Could it be Xbox One quality settings at 900p 30 fps? Or even PS4 quality settings at 1080p 30, or even 900p to 1080p 60 fps? We don't know.
Red Dead Redemption 2 is even rumoured to feature DX12 and Vulkan based on information
some people found in the Rockstar Games Launcher, this will certainly be of much benefit to PC Gamers as good implementations of these APIs can really make a huge difference for performance.
The inclusion of these will certainly make it one of the most fascinating PC releases to date!
Edit - Expanded post and made it clearer.