The special Reposition movement isn't "nothing", it's the special ability Pounce.
Here is a link to a similar question regarding Intervene.viewtopic.php?f=4&t=35152&start=10
Intervene - When declaring an Assist Reaction, this model may move up to 3 inches toward the attacker or defender. After this model Assists a friendly model, this model may move up to 3 inches."
Note that with Intervene, the reacting model gets to preform the "special movement" regardless of whether or not they get to actually preform an assist. Even though in order to trigger the movement, they had to declare the assist reaction. In this specific instance, the movement granted from Intervene is entirely separate from the assist itself, even though it couldn't happen without an assist being declared.
I see Pounce the same way. It can't happen without an attack being declared, but it is "not" the attack itself, it's a separate special ability.