What really excellent responses by both m1469 and Wolfi!
m1469, you're a sensitive and imaginative reader, and you got everything and more I'd intended with my post! Lang went first because according to throngs of devotees (egad!) he makes the most glorious, transcendent, transformative (didn't Kafka write a story about a guy being transformed into a cockroach?) sound in the world, and Lang brings his entire being, soul, expressionism, dress and emotion and utter and complete Chineseiness into the playing of the Middle C, and were his fans able to "snip" part of that Middle C sound wave and capture it like a clipping of Langs hair (or underwear) they would wear it around their necks forever and into the next life with the love and longing only Justin Bieber deserves.
Poor Mo, by contrast, is merely an unfailing servant savant who can reproduce the exact hammer speed that Lang made, producing the exact same sound, as evidenced in all manner of sound gadgetry and the testimony of white-coated "soundologists" the world over.
So, for sake of discussion, the two sounds were absolutley identical, and the soundologists -- utterly unbiased since they didn't know who was play the Middle C, nor did they see the performance -- concluded without equivocation that the two C's were absolute perfect identical sound twins.
Two humans -- with vastly different qualifications and lives -- making the same piano sound.
But if Mo had been a DiskKlavier Piano, or better yet a Welte Vorsetzer from a much earlier time. Would we be discussing the Vorsetzer's "soul" instead, and attempting to argue that there is more than key or hammer speed involved (aside from dampers), when in fact -- according to the geniuses who built the Vorsetzer -- there is not?
Listen and watch this, and you might be surprised at your reactions!<g> Quite astounding!!
And Wolfi, there is no doubt that "zillions of factors" go into the preparation of producing a certain speed on any single keystroke... like voicing, the act of playing one note in a triad for instance, a bit faster than the others to produce that "voice" which "sings" above the others.
Voicing is a bit of piano magic in that the ear conspires to with the finger perpetrator to fool us into hearing something that "we're not". A "voiced" G in a C Maj traid -- is it played at exactly the same time as the c and e? Does the ear knit them together since we "expect" a certain outcome?
A well "voiced" chord in a string of well voiced chords with greater and equally enhanced finger speed, played closely enough together and you fool the listener into "hearing" a legato phrase of singing chords making an even and steady crescendo.
If you go to the cinema and see a "motion picture" that has been done well, you will come out of the experience perhaps emotionally moved having related to the characters' predicament, made sad or happy or inspired by their story or what have you.
Actually, it is all magic in that it is a totally contrived illusion of flashing individual still pictures on a screen, which have been recorded under hyper controlled conditions, likely a number of times in "takes" by "actors" to make the illusion complete and utterly believable.
Unlike a visual art such as cinema, I see piano playing as much more pure in that it should require no visual suggestion to make its illusion complete. Though Brendel said we must suggest a crescendo through movement in an impossible situation like with one note, this is really another example of illusion and a bit of "acting" which is "cheating" a bit in my opinion.
IMHO, the quicker we realize what the piano is really about and recognize there is "no soul", the faster we will be able to create one.
Again, thank you both for your interesting repsonses!

PS
Wolfi -- Your English is Excellent!