I don't know if you're referring to me as the poster advocating intrinsic hand muscle use, but allow me to clarify my meaning.
To play the piano at an expert level, it is primarily the dominion of the fingers.
The correct feeling one must develop is that this generates from the hand and not the arms.
If one starts by attempting to substitute with the larger muscles outside the hand, then you're on the path to a very limited ability, IMO, since fine motor control cannot be obtained.
To say that I mean ONLY play with the intrinsic muscles is not what I mean.
We don't have direct control over which muscles we want to use for the most part, only the movement we wish to make which dictates the muscle(s) that will be employed.
So in reality I have no doubt that when we play with the hand muscles, the "out of hand" muscles are also active to some extent by virtue of the fact that they are connected to the hand and fingers and by virtue of the fact that we mostly cannot control muscles directly.
I actually do not know how much the extrinsic muscles participate when one plays correctly by primarily using the correct motions, posture, hand/arm position, etc. and feeling that the hand and fingers are the primary players.
To know this I suppose medical scientists would have to wire up hands and arms to an expert player with many, many electrodes and then record the electrical impulses and velocities of fingers, hands, arms, etc, and who knows what else to get an accurate and definitive knowledge of this.
But really, what use would it be other than to state with unequivocal accuracy which muscles are used and to what degree?
And I often wonder looking at the physical presentations such as your power point -- that they are missing something with regard to piano playing.
For instance, I have a question I've never found the answer to and that is: since the lumbrical muscles --unlike all other muscles in the body -- are connected to tendons instead of bone, specifically the flexor digitorum profundus and the extensor expansions, the "FDP" being your subject, I believe, which manipulates the finger tip joint... it seems to me that when the lumbricals are used by means of movement that employs them, that the FDP is also engaged by virtue of the fact that the contracting lumbrical is therefore pulling against the FDP's tendon.
See what I mean?
Perhaps you could find a qualified answer to this.
I'd like to know the answer since it always seems to me correct playing has a tiny bit of automatic finger tip control/participation.
I've never seen an answer or discussion of this regarding piano playing.
Seems to me rather the crux of the issue here regarding since you really cannot -- I'm surmising -- move one without the other.