No he said he wasn't 100% convinced that the key moves faster when producing a loud sound. I've been teaching for some time now as well. When students are required to play loud, I don't tell them "produce a higher velocity of the key". Things like dynamic markings can only be interpreted and applied in their musical context. That is a slightly different discussion. So I know the difference between what Nyir said and what you think he meant.
No, he read what I stated in explicit terms and accurately recounted it, based on consideration and understanding of my point. The only "difference" in anything exists between what I wrote and what you falsely assert that I said. Read again.
I indeed believe that fastest key speed doesn't always equal loudest sound. To accelerate something well you need to be continuously in contact with its resistance. It's been shown that when whacking a key, the hammer leaves contact with the mechanism much earlier. You lose the full range of movement in which they could have been coupled. In this case, absolute keyspeed may not be passed on- except to the keybed. A short bond between key and hammer doesn't give good acceleration, if the bond does not last all the way until escapement. Even if hammer speed were the only issue, as some assert in the tone thread, there's not a simple and unbreakable correlation between maximum key speed and getting maximum hammer speed. It isn't remotely so simplistic, or playing piano would be easier for all and not merely for the few. Simply move faster = bullshit advice, except for those with the technique to do it.
In any case, I ALSO said that thinking about speed is not the trick to imparting notable speed into another body. Would I only be able to hold one of the two views you listed? No. I hold both and they are not mutually exclusive.
There's good reason why cricket coaches don't tell batsmen either to achieve a big hit by swinging the bat as fast as possible, or even to try to make the ball move as fast as possible. It wreaks havoc with the coordination to have such an ideal. Moving your body fast doesn't automatically pass a large speed onto an external body. Often, the only way to pass on serious acceleration is to pace the acceleration in a manner where achieving a large absolute speed is not a conscious part or the intention, but the very thing you must hold back from allowing until the instant of separation (ie escapement) . When you learn to interact continuously with the keys resistance, you produce a huge tone without needing to attempt any feeling of speed. It's remarkable how slow it can feel.
Regarding musicality, I doubt whether many students are too unmusical to notice the five or so fs at the end or rachmaninoff's c sharp minor Prelude. Neither asking them to consider the musical context (as if they thought it just said single forte) nor telling them to move keys faster does a jot of good for helping an intermediate student to play with a truly big sound.