Reading about, in the world of synth Velocity seems to be how the sound changed in relation to the speed/force you push a keyboard button or strum a guitar.
What does it mean with regards to the PlayDate though? I can only think that it's related to the attack value of the envelope.
It still means how 'hard/pressure' you hit the note in the synth. Quite different to attack in an envelope. usually most notable change is loudness.
The penny just dropped.
The actual velocity is set with playdate.sound.envelope:trigger(velocity, [length]).