You seem to have misunderstood the concept of multipolygons. Multipolygons are designed to represent areas. The relation may only contain ways as members that form closed rings. At least one of them must have the role "outer". Rings within outer rings are assigned the "inner" role. In this case, the areas of the inner members do not belong to the area represented by the multipolygon. Therefore 
https://www.openstreetmap.org/relation/17355403 is not a valid multipolygon because https://www.openstreetmap.org/way/1246848233, https://www.openstreetmap.org/node/5148538660, https://www.openstreetmap.org/node/5148538661 and https://www.openstreetmap.org/node/5136318266 are not valid members.
In addition i think, the areas of the buldings are part of the industrial landuse, not excluded.
