We can debate whether "open" is the right word to describe what I hear uniquely from dipole planar loudspeakers as a class of loudspeaker. To my ears the presentation of a dipole planar loudspeaker is unlike the presentation of any horn.
This is suggested, if not evidenced, by the fact that dipole planar loudspeakers are typically and properly located 5 to 9 feet from the front wall to cultivate the rear wave, whereas horn loudspeakers typically are close to, if not right up against, the front wall. (Do you remember how offended was the Technical Director at the Avantgarde factory when I asked him why in a 40 foot long room the Trios were pretty much up against the front wall?)
There is no rear wave from a horn loudspeaker (unless, of course, the horn driver is designed that way and is open at the rear). It is a conceptually different loudspeaker topology.
I understand this, but to me the open presentation of dipole planars is something different than soundstage.
I agree if the attribute is bass or tone or resolution or many other things. But no upstream signal path leading to a box speaker can achieve the open presentation of dipole planers which I have in mind.
I agree with this statement in general. But some signal paths simply cannot deliver certain preferred attributes.
Now we are back to the original question. I disagree with this statement more than I agree with it.