That's pretty easy to test, I would have thought...

Play one note (any velocity), hold it. Play another while you are holding it. Does it play with the attack transients of the first?

There's your answer...

AFAIK, Yamaha's SA voice tricks are NOT based on velocity, but on whether you are actually holding a note down when you play the second... Play detached, you get the attack. Play legato, no attack. Velocity has been used long before SA, to get multi sample dynamics, or to trigger 'bends' or hammer-ons on guitars, etc.

The SA voice system also uses intervals BETWEEN notes you play to determine if elements like fret squeak get triggered. It's totally different from just velocity triggering...