(I happen to know one of the people involved.)
You cannot aim sufficient energy over distances like that
They were originally intended to be powered by betavoltaic batteries (solar cell sandwich with a charged particle emitter for the peanut butter - like the "radioactive diamond" batteries but with Strontium 90 for the radiation source). But another dude computed what local interstellar hydrogen looked like when treated as a proton/electron beam at 20% of light speed and concluded no other radiation source would be necessary - by a long shot. Just launch with the supercaps charged and you're up to power-generation speed long before they're discharged. You only harvest a fraction of that energy - the "solar cells" are far to thin to stop many of the protons but they make lots of electron-hole pairs on the way through - and you use heavy atoms (much charge per atom) in their semiconductor structure to maximize that. That gives you plenty of power to run the computer, sensors, and attitude control. Also the transmitters to phone home, with several watts total over the surviving portion of the swarm.
and you can't slow these gram-weight "robots" down with this propulsion system.
Sure you can. That not-quite-relativistic hydrogen wind through the radiation battery gives you enough friction, when combined with attitude adjustments, to bring the swarm into proper formation and all traveling at the same speed by the time it reaches the target. (You launch it over a considerable period, with the later ones faster than the earlier ones so they all arrive at the same time.) It's slowed a bit by encounter time, but not by very much. So it has to look fast as it flies by.