Multipolygons are a special kind of relations, they are not just borders.  They are technically a relation but follow the same rules as areas.  (Many buildings are multipolygons, even Belgium is a multipolygon in OSM.)

I understand that the purpose of this changeset was to draw it as an area instead of a MP.  I would tend to agree with that.  But it is true that "type=multipolygon" should not be used on a way.