It's a submarine, -70C means it's been frozen into a colder iceberg than exists on this planet.

I am curious if this is a mitigation for repeated stress cycles or an analogue of metallurgical properties in deep ocean water.

... or it's sitting at the surface at a very cold place. (yes, it obviously won't completely cool down to that temperature, but is exposed to temperatures at least close to it)

My assumption is that very cold temperature conditions may be a proxy for some other set of conditions which are classified. Do cold temperatures mimic something like neutron bombardment, in a way that would be metallurgically useful?

The short version is, no, there is almost no chance that this is related to anything exposed to a relevant radiation flux.