The P-38 can opener was widely issued by the US Army and Marine Corps from 1942 onward, and was small and light enough that it could be worn from a dog tag chain. Beyond that, it's not exactly rocket science to open a C-ration can (or a German iron ration can) with a bayonet or knife. You essentially sit the can on a flat surface and jab a series of holes in it. It's messier and more time consuming than using a can opener, but it can be done.

I have no idea where the idea that can openers weren't widely used came from.