Ballistic Missile Defense is irrelevant when the enemy has this: https://en.wikipedia.org/wiki/Status-6_Oceanic_Multipurpose_...
Consider, for instance, that there is no way in hell you could touch North Dakota with Status-6. If Russia wanted to attack missile silos in North Dakota, they would need to use their own ballistic missiles for it. Ballistic missile defense reduces the chance that they could succeed in any sort of decapitating preemptive strike. That it can't also stop a nuclear-powered torpedo is irrelevant; that's simply not what it's meant for. Other threats are/will be countered by other systems.
Hypersonic cruise missiles are not in the purview of ballistic missile defense, unless they have a ballistic stage, in which case they might be. Hypersonic boost-glide vehicles, having a ballistic boost-stage, are countered by some forms of ballistic missile defense, but not all.
Ballistic missile defense is a defense against ballistic missiles. Not torpedoes, cruise missiles, artillery shells, nukes smuggled in shipping containers, frogmen going up a river with a SDV, or anything else. Pretty much just ballistic missiles, or at least weapons with a ballistic boost stage.
I was responding to this, specifically, but did not elaborate enough: "in North Dakota, they would need to use their own ballistic missiles for it"
There are a lot more ways to fly a nuclear weapon to North Dakota than just ballistic trajectories. Bombers, cruise missiles, hypersonic missiles, etc.
No. That only means ballistic missile defense is not sufficient.