Yes, the promise the US made in that case lasted, in the sense that it didn't again sponsor Cuban exiles to mount a rebellion.  However, the US treated Cuba as an enemy pariah for 50 years afterwards. Short of invading, the US couldn't have shown Cuba more hostility. So all things considered, the promise alone isn't a very enticing carrot.

Practically-speaking, I don't see any means of normalizing relations between NK and other countries.