I'm wondering if this was already at the point where he'd started charging at them. Sadly we are getting video from another vehicle, a fair ways behind the event, and some of the most important moments are lost to bad phone handling or involved parties being obscured by the truck, etc.
I strongly suspect the guns were raised in reaction, not the other way around.
Definitely not in the frame in question. Travis raises the gun shortly after exiting the driver's side door. At this point, Arbery is still jogging, and is on the left side of the road. After the gun is raised Arbery abruptly shifts toward the right (passenger) side, increasing his speed and changing his gait. Travis then moves around to the front of the truck. As Arbery clears the truck on the passenger side, Arbery changes directions and charges at Travis McMichael, resulting in the fatal struggle.
There's really zero doubt about anything except whether Travis raised the gun. The cell phone photo on the internet is difficult to see. It sure looks that way to me, but I can't be absolutely certain. If Travis raised the gun, that's a threat of deadly force, and is a felony, and everything that happened afterwards is a consequence of that action. If not, then it might be worth taking a chance with the jury, insisting that there was no threat by the McMichaels.
Good luck with that. I think a reasonable person in Arbery's situation would have a fear of death or great bodily harm, making self defense a legitimate response.
People would also say that about Travis during the struggle. As Arbery tried to grab the gun, self defense would be a legitimate response. Unfortunately for the younger McMichael, he had eliminated the legitimacy of that defense by his earlier threat.
Unless they come up with something new, they're going to lose. If you are right, and they can convince a jury that Mr. Arbery was a fleeing criminal, then they might be able to convince a jury to let them off, even if the letter of the law says they should not.