An airship has more than one engine. In fact, depending on the design it can have at least 8, spread out over the hull. Depending on the design, these might be internally mounted(common for helium designs) or externally mounted in engine gondola's (Hydrogen designs use this for obvious reasons). These are fairly small, hard to hit targets, and protected (and obscured, in case of internally mounted designs) by the airship's balloon. (Sure, they can try to shoot the propeller, but that one is even smaller).
All this they have to do while under heavy flak fire from the airship itself. (Because well, your average airship has more than 32 cannons spread out all over the hull)
After all, if you hit a plane, it goes down too. And the planes are larger targets than airships's engines. And considerable less armed.
The airship's will probably be made out of Duraluminium alloys, combined with some of the lighter than air unobtanium. Thanks to the latter, we can probably use iron and steel too. I don't know what the balloonets and the like will be made from. Probably a combination of rubber, fabric, plastics and other supple and generally airtight materials.
Our current supplies of Autrefbium are 3 cubic meters. Duraluminum has a density of roughly 2000 kg cubic meter. The Graf zeppelin (Same size as a medium sized military zeppelin) weighted around 68 ton total. If we assume all of this is duraluminum (which it isn't.) We get a total need of 34 tonnes of Duraluminum. With those 3 cubic meters of Autrefbium that would mean a significant reduction in weight.