US boomers have enough firepower to wipe out everyone in Russia without the need for other weapons. That is the point. There is no 70%, it is 99.999%. At the end of the cold war a Soviet naval admiral said they had managed to track a US boomer twice for a few minutes.
The MAD balance is still there for the most part. The most part being it is unclear what % of Russian nukes still work given the shit show of maintenance and graft shown in the convention forces used in the Ukraine.
US has 14 Ohio class subs, ~90 warheads each. This is in addition to 400 MM3 ICBMs. Unlike the Russians the US knows their weapons system will work.