To be effective to 100m it would have to have near rifle muzzle velocities, and that would _not_ be "virtually noiseless" because that would be mach 2 or higher, and anything supersonic makes a loud crack. So if it was subsonic then the bullet drop at 100m would be at least a foot and a half. Wind would make its accuracy terrible enough, but then consider how you might stabilize a bullet made of ice... (I know it was a "dart", but the whole thing had to be made of ice or else plenty of evidence would be left behind.) And being an ice bullet (or dart) surely it would be melting along the way and not have enough mass to penetrate the victim's clothing.
No, the point blank range of such a weapon would be very short, perhaps not even 20m, and presumably it was meant to be used in public, which would be weird because the gunman would be very conspicuous.
I have no doubt they researched it but I highly doubt it was effective or used.