Mark Hamill is referring to how C-3PO is holding a bowcaster, the kind of blaster Chewbacca frequently uses.
There are other interesting aspects of this Star Wars: Episode IX poster, like what looks to be the other Knights of Ren and red-colored stormtroopers resembling the Revenge of the Sith-era clone troopers, but for now, it’s unclear if it’s a true leak or a fan creation.
The prospect of C-3PO firing on First Order stormtroopers is frankly hilarious given that aside from that time his head was attached to a battle droid’s body, the beloved, golden protocol droid has largely been a pacifist in the Star Wars universe.