To change them, you would have to create a file with the same path, but not directly in this mod, but first create a separate mod with a name like SoS_core.pak (I would name it that). The reason for doing this is that the mod is modular and divided into parts by faction. And if you want to make changes to the default stats for several factions and create the same file in other mods, and then someone installs these several mods together, the game will actually only read one of the default stats files. Namely, the one that was created last, if I remember correctly. And it will ignore the rest.
This file itself is of the XML type, and the creature bonuses are located in the combat/Creatures/Stronghold/DemonRage/Bonuses block.