As I wrote at https://news.ycombinator.com/item?id=16491298 , this is a completely predictable response to the US efforts to undermine the 'assured destruction' part of MAD.

The US has spent more than $1 trillion to upgrade its nuclear weapons systems, which seemingly is done to make it more effective in a first-strike attack to take out hard targets like Russian missile silos, while doing little or nothing as a second-strike response.

You should expect the Russians to develop methods to avoid antiballistic missile systems (which is much easier than deploying the crappy but expensive system we have), and to be at a higher trigger level - "launch on warning" instead of "launch on impact."

By comparison, consider the Russian "underwater drone able to carry a nuclear warhead." It is not a threat to the US nuclear triad, so does not prevent a retaliatory response from the US. What it does is allow Russia to make a second-strike response.