It is certainly a bluff. Accidents or misunderstandings aside, the only time the US chose to press the issue as it were was when it faced a serious threat to having its nuclear deterrent rendered completely ineffective (Cuban missile crisis). No one's launched nukes because of non-nuclear events in Korea, Vietnam, Afghanistan or Cuba, and no one will seriously contemplate it over losing pieces of eastern Ukraine or Crimea. Even taking the war into Russia proper won't do the trick, murder-suicide is only on the table when the threat is existential.

The bluff does, however, work very well in slowing down and dissuading Western help to Ukraine by cultivating the "don't do X else nukes tomorrow" memes and propaganda talking points. Very effective foreign policy tool for a nuclear-armed fascist dictatorship, other will certainly take note for future invasions.