Part of me thinks it might just be a mistake. The Base Game page lets you see "options" but if you pick the XS version it says "not sold separately" which seems like the spot the upgrade should be in if you have a digital copy.
The biggest issue (besides lack of 4 player split screen coop) is that the frame rate isn't unlocked in resolution mode (capped at 30fps). If that was unlocked it would basically be the XSX version.
The biggest issue (besides lack of 4 player split screen coop) is that the frame rate isn't unlocked in resolution mode (capped at 30fps). If that was unlocked it would basically be the XSX version.