I have seen very few measurements from non-environment rooms. I don’t want to draw conclusions from my limited experience with them. The design, as I understand it, makes little sense to me as a listening room. I would love to see your ETC measurement to see if there is evidence that the room is preserving the reflections necessary to produce as sense of envelopment. My hunch is no and the few rooms I have experienced did not. They produced an anechoic path of direct sound and then attenuated all 2nd order and higher reflections so extensively that the room would have been effectively anechoic at mid and high frequencies.
I think the idea was based on some good assumptions, but the few I saw in practice were too small to work. One of the principles is that there is a large gap between the initial direct sound from the speaker and the first reflection. For that to be true the room must not produce any very early reflections at or around the speaker. Those lateral first reflections are typically redirected behind the listener and then diffused and absorbed. At the mic position you effectively see an anechoic path for the first 20ms and then what might look like relatively diffuses sound. In practice it usually has no meaningful reflections at all. The proceeds of redirecting, diffusing, and absorbing in a moderately small space leaves the room dead.
there is also a fundamental problem with this approach in my view. The direction of the reflections matters to our perception of space. Reflections from the front, back, floor, and ceiling provide a largely monophonic signal. They don’t support our sense of apparent source width and are only a part of our sense of envelopment. Lateral reflections are key to that. Lateral reflections need to be relatively strong, not intensely mitigated, to give that widening of the source effect and greater sense of envelopment.
a more sensible way to make a room sound bigger than it is would be to simply use a multichannel setup. In the absence of that possibility, diffusing lateral reflections can create greater phase randomization of the cues which better mimics how a larger room would be perceived and gives a similar sense, but is an imperfect solution itself.
in my opinion, the non-environment room isn’t one that makes a lot of sense and ultimately misses the forest for the trees.