I think we're confusing realms/loka/planes with physical dimensions (except you, Walter). When Robert talks about the dimensionality of negative entities, he's not talking about what dimension they're coming from, but whether they occupy space or not- which requires three dimensions (10 if you follow string theory). This is why barrier methods work- because earth elementals (which are the main ones he has spoken about) cannot exist 'in the air' and need a host to get into- because they don't have the capability to manifest in third or fourh dimension.
To recap: Dimensions in spacetime are: length, width, and depth. Those are three dimensions. And the fourth one is time.
Negative entities are two dimensional, like a plane- they can't just go off on their own because they don't have volume, don't occupy space.
And yes, I remember when we talked about this, so many years ago.