Here is where I think I will step in.
You are contradicting yourself, or at the very least being unfair and contrarian.
Since he wrote honest Materialists will have to admit to be wrong on the nature of reality itself depending on actual, observable data if anything contrary to their current paradigm shows up to be real and they cannot explicitly explain it inside the current paradigm, he devised a thought experiment, much more believable in-game : Two scientists, one Materialist, one Spiritualist, do a science conference together. Both will then make a case, in-game, about their claims about reality. As it will occur to you, in-game, only one of them can do something observable the other cannot do, and this is the Spiritualist. One must understand that at the very moment there is actual observation possible of esoteric phenomenon, they stop right there to be pure fantasy and enters the world of reality. The reason “esoteric sciences” are discredited is there is nothing observable about them ; they are more or less based on faith. Is there something as the Ethereal Plane, or the Spiritual Plane, or Philosophical Stone, or Morphogenetic Field, or whatever else of the same ? If yes, then the thought experiment tells you the following : If there is anything they CANNOT understand inside their current paradigm – materialism -, then it follows logically they are wrong. You admitted yourself when you wrote : « They may eventually have to amend the claim there’s nothing more than matter but that doesn’t mean they are suddenly spiritualists. » It is true though, it does not make them Spiritualists ; it means they cease to be Materialists IF there is anything else BUT matter. That is the point if you say there is another plane of existence since it would mean there is something else besides the Physical Plane.
The best possible outcome at this juncture is to pretend Materialists will work in-game on this phenomenon and try to find it a materialist explanation, which is highly probable, especially for Fanatic Materialist, but because of the confines of the game, they cannot and they will never be able to, UNLESS they become momentarily Spiritualists. Until then, it will be FOREVER useless to research it as a Materialist.
That is why, in-game, Spiritualists seem to be right all along ; there were always things out there beyond the Physical Plane.
It is what Aono is trying to tell you, I think.