Yes, indeed it’s a typo. @michalis
Overall, Castle Game Engine uses URLs for most operations. I.e. you can still use
Scene.Load('car.x3d'); in case
car.x3d is in the root directory of the project (will be converted into
file:///car.x3d. But usually all the assets should be located inside the
data folder as that article recommends, and then they are accessible through internal
castle-data:/ protocol which also guarantees that the Engine will find the assets on different platforms if they have to be moved to a different location, e.g. on Android and iOS.