The concept of brandishing is well understood in the criminal system, even in states where open carry is legal. Intimidating someone through the display of a weapon is understood as an assault, like you say.
Brandishing includes a variety of behaviors short of actually pointing a gun at someone. Actions that a reasonable person would understand as a intimidating display of a weapon is considered brandishing. Something like simply moving a hand to the butt of a holstered pistol, or, I dunno, demanding someone to stop while holding a shotgun at the ready, is well understood to be a brandishing of weapons.
Setting up a roadblock for the sole purpose of confronting him while openly armed is something the DA totally elides in his explanation of not bringing charges. He frames it as inexplicable event that Arbery might attack the armed man, not mentioning that this was the second time that they had attempted to block Arbery with their truck and stop him. Shotgun man got out because the first time, Arbery kept running.
The police report, by the words of the killers themselves, is quite clear that they forced this confrontation.
https://int.nyt.com/data/documenthelper/6915-arbery-shooting/b52fa09cdc974b970b79/optimized/full.pdf
