I've just changed the gate to access=no it should make it impassable.
The rest of the roads are marked as private (exactly the same as those around Włodarzewska 57C). It means that the road is not to be used by the general public. Access is only with permission on an individual basis. 
Routing between two locations where there is no public road between them might be weird or incorrect. OSM data is correct in this place. You should contact Graphhopper/Valhalla developers to investigate this issue further.