text: "A MultiPolygon is a MultiSurface whose elements are Polygons." So by definition this is a multipolygon.
  
  Also type=building was proposed in 2007. 13 years later there are less than 100 objects using that tag in Hamburg. Which means there is no reason to use it at all. It per se means it's deprecated.
  
  For this building it doesn't make any sense to use more than  a simple way anyway.
  
  And the shape isn't even correct the one we see on the image in editing mode.
  
  I mean all this is also about people using the data, which simply doesn't get easier when people's "mapping fun" gets somewhat out of hand.