It's worth considering, but security personnel do sometimes have hand signals as normal operating procedure for a litany of reasons, like not cluttering comms, not creating undesired noise (he was right next to Kirk who was speaking in public and taking questions), not competing with ambient noise, and being understood over distance without needing to again cause one of the aforementioned problems. Most security people are going to be formerly military or police, where such signals are commonly used (or at least taught, in the case of cops). It's also part of the "operate" aesthetic that those piggies love.
While it narratively looks right to have a dude signal for the attack right next to the target, like Judas betraying Jesus by kissing him, I don't think there's really all that much reason for there to be signals down to a "fire now" from someone right next to Kirk when the event is highly public, has many uncontrolled cameras on it, Kirk mostly isn't really moving during the presentation anyway (he was sitting down), and we don't have reason to think that exact moment is an especially good time compared to 30 seconds earlier or later, plus the dude was not trying to conceal the gestures at all. It would be absurd for someone to blatantly produce a hand gesture that was covertly to tell the shooter to fire when he was so close to Kirk and therefore sure to be caught on camera like he was.
Overall, I think it's extremely unlikely, though again it's a question worth asking. I agree with the other commenter that being startled isn't a strike against the theory, but there are too many other factors against it.