pianoloverus the researchers looked at acceleration at the point of escapement (which I think is closely related to attack speed so the difference seems inconsequential)
In fact the difference is very important. It's perfectly possible for two objects to be moving at exactly the same speed but with different accelerations: one could in fact be speeding up while the other is slowing down.
Until the moment of escapement, the pianist is in control of the acceleration. One pianist might start with a high acceleration by striking the key fast from a distance, then applying less force and consequently producing less acceleration throughout the rest of the motion. Another pianist might start with low acceleration (with the finger already resting on the key), and then increasing the force progressively. It's possible that just at the split second before escapement, both pianists have accelerated the hammer to a speed of 3 m/s (a typical value for a forte), but in the case of the first pianist the hammer is no longer accelerating, whereas for the second pianist the hammer is still undergoing considerable acceleration.
If the string was struck at this moment, it would not be unreasonable to expect that the sounds produced by the two pianists, although being at the same volume (due to the hammer hitting the string at the same speed), might differ in timbre because of the difference in acceleration. But the string is not struck at the moment of escapement; the last part of the motion of the hammer is in free flight, which means that the pianist no longer controls its acceleration. From the point of escapement until the moment the hammer strikes the string, the only acceleration the hammer is subject to is that of gravity (-9.8 m/s/s).
So, although these two pianists produce a different acceleration at the point of escapement with the same hammer speed, in both cases the hammer has the same speed and the same acceleration when it hits the string.