Hot take: It's been in development for 20 years, everyone is just now starting to freak out because holy shit it's real.

More realistic take:

The weapon is viewed primarily as part of the nuclear deterrent acting as a Second Strike weapon hitting coastal populations centers like New York and Los Angeles. In this scenario its lack of reliance on satellites and the fact that it literally goes underneath missile defenses make it a slow but inevitable death. Critically this is a diversification of Russia’s nuclear deterrence and not a replacement for submarine launched missile; Russia is still planning to completely replace remaining older ballistic missile submarines with the new Borei-A (NATO: DOLGORUKIY Class) boats.

Second-strike capability is any weapons system that allows you to make your enemy regret hitting first. American capabilities have always been focused on first-strike.

The Soviet Union was never confident in its first-strike capability. Soviet (and later, Russian) doctrine has always been about make the cost of American aggression so steep they'll keep their missiles in the silos. So while Putin has definitely been painting this thing as the weapon by which the Americans will be cowed into acknowledging Russian military dominance, much less ink has been spilled about how we'll all be dead by the time it hits anyway.

TIL there's a seabed treaty.