I found a way to try all of QuickBMS's decompression methods on a file (which seems to be 500 different methods), and not one of them decompresses the DDS files to actual DDS files. If none of these 500 worked, then the method for compression here must be very rare and probably unique to the game.
I hope Sam's friend manages to do better than me.
I hope Sam's friend manages to do better than me.