I'd argue that the combat-relevant definition of robot is any machine that can perform some combat duty that, if blown up, will not injure or harm a human, only equipment. How simple the machine is doesn't matter, as long as it's out there and a human isn't, but up to this point not very many things have met this criterion.
Certain craft flying autonomous patrols and are capable of rudimentary target acquisition, although they must ask permission from a human operator before firing. (And yeah, I'm in a position to know).
From what I can tell, the ground based bots don't have any onboard intelligence - they require an operator at a remote terminal at all times, more like telepresence than autonomous or even semi-autonomous robotics. In contrast, the UAVs presumably have the equivalent of an autopilot, and the X-47B referred to in the article appears to have a very good one. The equivalent ground-based robot would probably be something like a backpack scout that you could chuck into a building and have it autonomously map it.
Definitely the X-47B could be called a robot (or an autonomous, mobile robot if robot takes on the broader definition used by a few of the other commenters). I wonder how it reacts if it is engaged or damaged. Does try to self-preserve by evading or moving to a safe place without human interaction?