In class I too sometimes compare what we do with other styles, but ALWAYS say it is not correct for us to it that way, never that they are doing it wrong only different (immediately followed by: "depends what you are looking for"). Students watch vids on youtube and have questions, rightfully so. I explain, show the differences. As for IS I am not so sure. Some have referred to vids of Ueshiba that show 'his' IS. All I see is good posture, good ma ai, kimusubi, kino nagare, etc. But that is just me, no judgement here (again to each his own).
Let me dissect a little microcosm you pointed out there, good posture. Let's say that, a couple years ago, I too would have said there is such a thing as good posture. I would look at just about any high level martial artist, notice something about their movement, and call it posture.
But now, after acquiring a certain different perspective, I no longer see posture where I once did, or rather, I see (forgive my embelishment) bad martial artists using posture, and good martial artists completely lacking it. I now see posture representing a fixation that can't adapt or move, it's a mind that is dead and a body that is dead. Before I trained to have a posture, now I train to have none but instead to move, everything, in unity of purpose, at every moment, with that purpose constantly changing, never stuck.
But from an outside perspective, what I used to label my posture now seems to look better to me, if only I take an artificial snapshot of my movement. But even that mental snapshot is an illusion, solely an artifact of the limited view - Shroedinger's posture, maybe there, maybe not, or perhaps Heisenberg's posture, if you know its shape, it is not moving, or if it is moving, you don't know its shape. Viewed across the totality of the movement, there are an infinite number of postures, so no need to pick any one out and give it signifiance, there are plenty more where those came from. But yet it is not something that just happens, it requires deliberate practice to happen.
Maybe you found a more enlightened perspective on posture in aikido than I did, but well, I never found it until I ventured outside, but that is another reason I did venture outside. It was a thing staring me in the face all along, I just did not know how to see it, until I was shown how.