It's possible that warwick was actually stunned, but since he already activated his ult, the stun would have done nothing.
This is because stuns do not disrupt abilities. So say me being an Elise player, I use my Human Form e cacoon to try to stun a thresh during one of my ganks. If he uses his Q to start a hook animation before my stun lands, even a second right before the opposing Thresh's ability is activated, my stun will not disrupt the hook, making my stun almost useless, because Thresh was locked in the animation anyway.
The same concept applies to warwick ult. His ult cannot be and was not disrupted by your parry; the animation for his ult is much longer than the duration of your parry stun, and the Warwick ult was activated before you could even stun him with any ideal timing. Ergo, if you stunned him, your stun from the parry would have been mitigated.