Totally untrue. Zahn never met GL at the time, and he recently commented on what little he may have heard about the actual ST: http://moviehole.net/201259395author-timothy-zahn-of-heir-to-the-empire-talks-star-wars-episode-7 He does say "as I understood it", so anything about Luke's children can hardly be taken as definitive - but he does, effectively, say that his own trilogy was unrelated to Eps VII-IX. The Dark Empire series, however, is something of a different matter - GL was apparently a bit more hands-on with it, plus he supposedly provided Kevin J Anderson with background material regarding the Sith for the Tales of the Jedi comic series, which was a spinoff from DE. Most likely just old drafts of SW, but one has to wonder just how much he might have let slip about the post-ROTJ SW universe while involved in just such a story, and what might have even been included. Probably nothing, but you never know.