That seems likely, although I strongly doubt the requirements are that stringent. The issue with bombs isn't just that they release (and create!) a lot of radioactive material, but that they pulverize that material and push it high into the atmosphere where a lot of it spreads around the world. Whatever radioactive material might have escaped from something like Chicago Pile 1 would have stayed local.

Supporting this idea, here's another article on salvaging Japanese wrecks which mentions low-background steel in the context of ships built in 1943 and 1944: https://www.theguardian.com/world/2017/feb/09/images-reveal-...