rectangular models are still cut correctly even if they are not closed (like the doorframe without threshold)?

No, there's no exception at this time, and the models without a threshold need a doorOrWindowCutOutShape# property set to M0,0 v1 h1 v-1 z. Anyway, for best performances, I set the doorOrWindowCutOutShape# property to this value for all existing doors and windows in Sweet Home 3D and its furniture libraries.
No value would mean "compute the cut out shape from the 3D model", which would be a useless loss of computing time.