An SS-21 can carry a nuclear weapon, but they also can carry conventional ones.
This appears to be more posturing to me. This is rapidly turning into a new “Cuban Missle Crisis.”
Agree with the posturing, these are not very effective in a cobat significant standpoint, and I doubt any US commander would give them much notice. They only carry a 1000 pound warhead, not even equivalent to a good 2,000 pound bomb with guidance. I would take the bomb any day.