Well nobody said whether it should be officially supported or not. Technically, the engine itself is compatible and also determines the savegame directory with respect to lack of dedicated saved games folder that is usually there on Vista+.
While OpenGL is a bit alien to me, if search results are correct, the engine uses OpenGL 3.2 features, which is rough equivalent to Direct3D 10, which is beyond 9.0 that XP supports. I imagine there aren't many games that use GPU features that, when accessed through Direct3D, are only accessible on Vista and beyond. So that's why I find this game particularly interesting.
Similarly, Wolfenstein: The Old Blood runs on Vista, despite "requiring" Windows 7 officially. The bumped up requirement relative to previous game could be artificially imposed by targeting Vista with used compiler alone, rather than explicitly depending on some Vista+ feature, though it's not something I researched. Getting newer things running on older Windows can be painful.