technology

Those are two entirely different things. Railguns are a sort of "this could scale to an absurdly high level if you can pump enough current through it, but at those levels it oxidizes its rails rendering them useless after a few shots" thing (and the big artillery pieces that would theoretically enable are kind of dead anyways, since missiles do their job much, much better), while coilguns are just a projectile firing version of the EM catapults that some carriers and roller coasters use as launch systems. Making a good coilgun is comparatively well understood, and it just hasn't been weaponized because energy storage and all the hardware for it has generally been heavier than conventional ammo and a firearm that's able to handle the pressure of a given shot.

Here I'm assuming they've just optimized the firing sequence to minimize the waste and are using high end modern batteries to get something with the size and weight of a typical infantry rifle but the performance of a machine pistol, for the explicit purpose of reducing its volume as much as possible. So it's kind of comparable in role to something like the Soviet's AS Val/VSS Vintorez rifles with their integrated suppressors and subsonic cartridges.