"I did see-- that." The pause is to twitch at the sudden introduction of fist into nothingness, in their direction. They blink at it owlishly. "No. Fistbump?"

It sounds fairly straight-forward, though, and probably not actually dangerous, so they hold out their flesh hand and make a fist, mimicking Sora's gesture. Not without immediate touching distance, in case it's viewed as a threat somehow. Even though Sora wouldn't really be thinking someone mimicking him is threatening, probably. Better to be safe.