We've had loitering munitions that choose their own target autonomously for a long time, for example anti-tank weapons that climb up after being released from a plane or helicopter then sit on a parachute until spotting one or more tanks and firing warheads at them.

The superficial new thing here is the exact quadcopter form factor, but the significance is the new price point. You bet the loitering anti-tank weapon costs a fortune. These drones are very cheap.

Of course, mines can be even cheaper, but you unwittingly engage them rather than them engaging you.