Regarding the first sentence it seems that it would automatically run in DX9 if played on XP. At least, that is what the wording alludes to and the Unreal Engine 3 was designed around DX9 so XP users could run it.
And DX11 is not better optimised yet, beteleguzah, Civ5 is a good example as are most games with DX11 features which hog system resources.
Besides, as has been pointed out before...if SE is indeed running XP then they only have one option.