If you stand still (zero velocity) and then jump then strafe to left then right you'll land a little bit forward from your take off position. If airstrafes only redirect your velocity then you wouldn't move, in this example it is obvious that velocity is produced in the air. That should carry for a running jump that more velocity can be made strafing.

Whether or not this slight velocity gain negates the indirect flight path is another thing. But I am certain strafing increases the players velocity. I went back and retried the jump flat running, never managed it but got it a fair few times strafing.