Delivery of nuclear weapon via shipping container might seem like a deterrent but it's kind of the opposite thing.
For something to be a deterrent it must have a few properties. Delivery taking a non-zero amount of time and producing a gigantic visible ordeal from outer space is a feature here. A container bomb going off somewhere in a civilian logistics chain is a surprise. Surprises cannot be deterrent by their very definition. The inability to ~instantly attribute the attack to some party would only invite additional instability.
Container ships tend to be fairly slow to respond and may not function as expected during a nuclear war.
The only way for this to work as a retaliatory measure is to have the weapons already in place at the target locations. Now, imagine if someone were to discover the weapon and trace it back to whomever installed it. This is effectively a slow motion nuclear exchange that was initiated by the "defender".
A key feature of those subs is that it won't be six months later. It will be an hour later because one is already stationed just outside your waters.
It's also a bit more sneaky than a damn merchant vessel. You really think you're getting secrecy of a nuke existing on a merchant vessel? Why? You have given the enemy intelligence agency nothing more than an entry level homework assignment. That vessel is 99/100 getting intercepted or sunk. How many of your merchant vessels are otherwise sailing towards the country that just armegeddon'd you?
"Twice in recent years the two of us helped an ABC News team that smuggled a soda can–size cylinder of depleted uranium through radiation detectors at U.S. ports. The material did not pose a danger to anyone, but it did emit a radiation signature comparable to that of highly enriched uranium (HEU), which can be assembled into a nuclear bomb."
> How many of your merchant vessels are otherwise sailing towards the country that just armegeddon'd you?
For something to be a deterrent it must have a few properties. Delivery taking a non-zero amount of time and producing a gigantic visible ordeal from outer space is a feature here. A container bomb going off somewhere in a civilian logistics chain is a surprise. Surprises cannot be deterrent by their very definition. The inability to ~instantly attribute the attack to some party would only invite additional instability.