PTS1, I actually think that your primary point is complementary to Marik's (though, it should be noted Marik was referring to projection, not tone).
For the sake of argument, let's accept your premise that individual pianists cannot produce an individual "tone" or "sound" on a piano, when comparing the playing of a single note. I think this is a good starting point. Though I am not familiar enough with the science or counterarguments to commit to this point, by comparing to the situation with other instruments, as you do in your post, I have always thought that the potential for drawing a different "tone" from a single note on a piano is negligible at best and the debate is, to my mind, academic. However, I also think it is clear that when different pianists play an actual piece on the same piano, the pianists can make the piano sound differently, and these differences can be significant and obvious.
If the pianist has no control over the sound of each note they are playing, aside from loudness, then to what do we attribute the differences? I would say that there are a number of contributing factors. Things like voicing: playing one voice quite loudly and all others quite softly can give the illusion of a "thin" tone", for example. In addition, a pianist who adjusts tempo, spaces between notes, type of articulation, pedalling etc. to suit the acoustics of the hall (size, amount of resonance, etc.) can sound like they have a clearer tone, or a richer tone than a colleague who does not so adjust. A pianist who has a good understanding of the strengths and limitations of the individual piano can sound better than the pianist who doesn't (if the piano sounds coarse when played too loudly, then scaling back all the dynamics proporionately in order to avoid playing at that volume can make a pianist sound like they have a more pleasant tone than their colleague). This is leaving aside the very dubious comparisons of pianists using different instruments, of course. Similarly, playing in a way that is unstylistic to the piece at hand, so to speak, can impact a pianist's tone: failing to taper the dynamics of a phrase where called for can make a pianist sound "harsh", for example. I could go on but this is already too long.
The point, simply, is that one can accept the basic premise that tone is not variable and still believe that pianists can "sound" very different from one another without resorting to mysticism, or wishful thinking. The concept and perception of sound quality is a complicated matter, and I don't think it follows that inability to vary a single note leads to the conclusion that the same piano will always have the same "sound", regardless of who is playing or how they are playing it. I have no idea whether or not Marik accepts your premise, but it seems to me that his post sets out a bunch of practical observations on the factors that contribute to sound quality on the piano (which is what I have tried to illustrate). Unless I am missing something, I do not see in his post any reference to national "schools" of piano playing, arm movements, reputation, clothing etc.