The only parts that really need to be metal are the barrel and bolt to contain the burning of the powder, and springs. Although, you can get by with plastic barrel and bolt if your design is sufficiently strong, and you don't have to have non-manual reloading, obviating the need for springs. The silencer's important parts are simply pressure baffles that don't need to be metal (although it helps significantly).
In this case, the design was replicating that of a Glock 43 IIRC, which is already mostly plastic. Guns are modular of course, and so there's a part that's legally designated the firearm to avoid ship-of-Theseus problems. For the Glock that's the frame, which includes the handle. That part's made out of plastic (except for a metal plate that has the serial number and can be detected by metal detectors). Since the frame on the original gun is plastic, it's designed not to have much force put on it, so it's not a technically difficult 3d print either. In the US (federally; states may have further restrictions) it's not illegal to manufacture your own firearm as long as it's for your own use. All the other parts, like the barrel, trigger, etc (not the silencer though), are unregulated, so it's perfectly legal to 3d print your own or just buy them and have them shipped to your house. (There's also the concept of an "80% lower": at what point does a piece of metal become a gun? Generally it's held to be that before the gun's than 80% complete then it's just a piece of metal, so you can buy The Glock uses a tilting barrel system: the barrel and slide are joined by a set of lugs on the barrel that fit into recesses in the slide, and after the bullet is fired are pushed back together by the force of recoil. The slide slides on linear rails, but the barrel has a pivot, so eventually as the barrel and slide travel back together, the barrel will tilt out of the recesses in the slide and allow the slide to continue backwards, opening space between the barrel and slide for the previous round to be extracted and a new round to be loaded automatically. You can imagine that hanging a big heavy tube off the barrel in the form of a silencer can be detrimental to the working of this system. In fact, you're supposed to use a recoil booster to increase recoil while using a silencer on tilting-barrel pistols to ensure more reliable operation. It's unlikely that malfunctions were because any parts were 3d printed in this case.