There are both.  However having one point/poi with building yes and a way/polygon with building=yes constitutes of two building objects and there's only one on the lot.  So one or the other but not both.

However it's okay to have a POI/point without building=yes, then it's just an address object which doesn't conflict with the way/polygon building object.

It is true that parcels with no buildings on them with a POI object is fine.  And one can dump a building on it and it's still fine though the building might not have the address unless it encompasses it.  Just is a problem when there are two building objects on the map when there is only one on the ground.