Maybe it's a Katra thing.The objection to Mrs. Spock and offspring is that they might be too far in the future. If that's the case there were most likely corpses. (Why are THEY not being reunited with Spock before he dies? Who knows? Maybe they have to go see HER parents? ... Er, the Vulcan one.)
Or time travel. Or Q. Or some inter-dimensional hoodey do that is unique to this story.