Is the barrel still rifled? If so and the bullet is spinning how on earth is it able to steer itself in flight?!? Amazing!

I'll bet there is no rifling on the bore of the weapon it is fired from, unless the electronics are giddiness proof.
My guess is that the target acquisition is by infra-red and the three rings in the middle of the round effect the steering. I imagine that it would take something like an A10 Warthog (https://www.youtube.com/watch?v=1rGcn2XGr48) to cope with an enemy equipped with these and as far as I know there is nothing like an A10 Warthog.

...and then they release a multi target lock, modified gun for the A10 that fires these.
And the Chuck Norris of Airplanes is born.

War just became a little bit more asymmetric.

Amazing technology. Imagine if they could use our taxes to help people and the planet instead of killing them and destroying it.

A very promising weapon for snipers and commando units with adequate backup.

The weapon is still rifled. It's easily possible to put a ring laser gyro and accelerometer inside a round of that size and perhaps use some sort of small motor to affect the bullet's path by varying the motor's spin.
The laser sensor, guidance/steering components, and power source would need very little space and weight is not really a concern since they'd weigh less than volume of metal they replace.

Ah, bullets, guns, awesome! Let's all keep believing in the fairy tale that shooting people solves problems instead of perpetuating them. Evidence be screwed.
And no, I'm not talking about a hostage taker in a kindergarten. I'm talking about warfare and the military's way of thinking.
And yes, that latter case is what this is developed for, since the kindergarten hostage taker problem is too small a target market to dump millions of dollars into.
First, the U.S. military will be the only one who has this. Decades later, it will have trickled down to the last roof dwelling sniper defending some backwater tyrant by shooting pedestrians.

As a retired Ordnance officer I would expect that the next major system developmental step would be to mount this weapon to the underside of a small drone with a very muffled engine. It would be great to be able to punch a target's lights out from a mile away, at some modest altitude, all without putting a team on the ground and all of this from potentially several hundred miles from a support base or maybe even a distant larger "mothership" carrier/launcher aircraft. New mission for a C130? Cruise missiles are a big expensive hammer. Joint Standoff glide bombs are still a big hammer with lots of potential for collateral damage. It would be great to be able to cancel someone's ticket from a hundred miles away and only spatter the wall while leaving virtually no other signature.

The DARPA web page for the EXACTO mentions aero-actuation controls, which implies some form of fin. But no fins can be seen in the illustration. The optical guidance system has to fit in the 1/2 inch diameter bullet, so it's probably simple. If optical refers to a laser, then the laser beam could be detected before the projectile hits. An ordinary 50 caliber bullet takes over 2 seconds to travel 2000 meters.