Poseidon and Skyfall are two distinct weapons with a shared motivation. Skyfall is a nuclear ramjet powered cruise missile, while Poseidon is a nuclear powered torpedo (in principle an unmanned miniature nuclear submarine with a suicide mission, although probably without hydrostatic buoyancy control.)
Both are motivated by the perception that ABM technology may render ICBMs ineffective.
Old news. Submarine-powered cruise missiles have been for decades Russia's biggest defense weapons against USA' offensive threats. You cannot just defend them. Like the Granit (which most likely was shot at the Pentagon)
Those two nuclear powered missiles are not much different, just they can be theoretically be shot off days ahead. The submarine must not hide for weeks near the US coast.
ICBM's are unneeded since the 80ies. That's why they don't have them.
To be clear, a Poseidon is not a cruise missile; it's a torpedo. It has different characteristics from an Oscar class loaded with Granits. Being a torpedo (which doesn't fly) a Poseidon can only attack things on or next to the water (probably ports / coastal cities.) Because a Poseidon is slow (relative to a missile) it seems an unlikely choice for a first strike attack; it's more likely to be intended as a retaliation weapon that's hard to intercept. Russia may also be planning on creating Poseidon "silos" on the seabed of the Arctic Ocean. Possibly related to this, Russia has a single submarine capable of carrying a Poseidon, the K-329 "Belgorod" which is a stretched Oscar-II. It can also supposedly operate as a mothersub for small submarines like the Losharik; the plan may be to use a submarine like the Losharik to install Poseidons on the bottom of the Arctic Ocean to act as a 'Dead Hand' system. However after the Losharik fire, I wonder what their plans are...
The biggest advantage of a Skyfall missile over a Granit is obvious; range. If a Skyfall missile works as they plan, there'd be no need to sneak an Oscar anywhere close to the target, or even use a submarine at all. Furthermore it could strike inland targets beyond the range of a Granit fired from an Oscar sitting in coastal waters. An Oscar would need to sneak it's way up the Mississippi River if it wanted a chance of hitting a target in Nebraska with Granit missiles, which leads into:
ICBMs. With respect to ICBMs you're mistaken. Russia does have a variety of ICBMs and SLBMs still in service with plans for more. They have enough already to easily overwhelm America's extant ABM systems, which are primarily a defense against smaller nuclear threats (e.g. North Korea.) Poseidon and Skyfall are hedges against the possibility that ballistic missiles are rendered truly obsolete by ABM systems, but that scenario hasn't [yet] become reality.
Both are motivated by the perception that ABM technology may render ICBMs ineffective.