Well, perhaps fallacy is too harsh. But I would consider it to be an over-generalization. Inductive nor deductive reasoning can't take one fact involving a single variable in isolation and generalize on it when there is clearly a host of interacting and confounding variables present in the inquiry. That be as it may, that's why I mentioned the pianist as illusionist using all the many resources of the instrument and artistry. In all the many other instruments in a symphony orchestra, I believe that the piano is absolutely unique in this respect, especially for a percussion instrument. For me then, Ortmann's finding that he could produce a decent tone with the eraser end of a pencil is mostly irrelevant when I listen to a beautiful rendition of Ravel's "Ondine". I do accept that hammer velocity is the key to volume, which is definitely useful and actionable information. I believe it might have been Josef Lhevinne who used to encourage his students to think of raising the hammers, not depressing the keys in achieving dynamics.
But why would the fact it's a combination mean that the parts are not also of interest? In a recipe, the taste is the product of a big whole. But wouldn't you be interested to know whether a nicer tasting one was made with fresher ingredients, or whether it was down to a different blend? If you cook with rotten meat, no amount of blending gets tasty results. It's compromised by a disgusting component. That combinations are variable does not mean that there's no interest in whether the constituent parts have an inherent quality. When listening to Kissin in a concert hall, I've often been certain that I could hear the thuds in his playing. I could actually single them out from the sound of the strings. I'm certain it went beyond relativity. Some tests have supported the notion that thuds are audible- regardless of what some claim has been "proven". It stands to reason that playing many harsh notes would amplify the inherent harshness further still, compared to a lone note. Also, I read somewhere that Ortmann's findings are extremely dubious based on the evidence he took. It seems he was very liberal about deciding that recorded spectrums were "the same".
My own way of dealing with this truth is to take advantage of every opportunity to rest the hands in the score. These are the lift-offs of the hand at the end of a phrase, observing a rest, fermata, ritardando, etc. I grant you that there are some pieces that are relentless and might not offer such opportunities. Your hand tightening technique is effective as it clearly works for you, but again reminds us that fingers always need to be taut, never droopy. The wrist though must usually be flexible (forearm octaves being an exception), and the arm mostly relaxed when not initiating deliberate movement.
I'd never refer to it as tightening, myself. It's always based on movement- not bracing. I do need to retain enough activity after the movement has been completed- in order to prevent drooping. However, even here I like to relate it to intent at movement- which largely means the intent to straighten the finger (although only just enough to balance, rather than actually cause further movement). I find this very effective at focussing a small effort into producing extreme stability- without any sense of stiffness. It's like when standing. You don't think of stiffening to stop gravity buckling your knees- you just push yourself up until balanced. Also, I'm increasingly realising that the arm can over-relax. My habit used to be vastly too much release in the shoulder- allowing my arm to slump in far too lazily. I actually had to a lot of work to get used to balancing it better- rather than keep relaxing to excess. I still have to be careful when relaxing, not to over-do it.