Leaning/Appoggiatura

Are we sure that the arm commands and not the fingers? I am referring to the part:

But what matters most (and here lies the point of the question raised) is that by properly applying the weight of the arm, one can vary the sound even between the fingers of the same hand. This becomes necessary when, in cases such as the one in the example in question, the composer requires playing melody and accompaniment in the same hand and thus, implicitly, requires two different sonorities in the same hand.

Perhaps I misunderstood, but the crux of the matter seems to be (also in general) the arm. I am being taught that more "parts" of the body participate, and in the specific case of polyphony—see, for example, the Bach repertoire—I am not certain, from self-observation, that the work is done by the arm.

Are we sure that a certain technique/"approach" has unique validity for the entire repertoire?

I mean, is it plausible to use 'appoggio' in baroque, in classicism, in romanticism... in the '900s?