My guess is that, for the most part, the late talkers described in the book tend to be VSLs (possibly with some left-brain weaknesses).
There is a whole yahoo group devoted to this subgroup of late talkers (as described in the book). The last time I was reading it (quite a long time ago) there seemed to be a lot of effort spent on getting a correct diagnosis, and fending off incorrect diagnoses (usually of a spectrum sort).
http://health.groups.yahoo.com/group/NaturalLateTalkers/ I have no idea what proportion of these kids turns out to be gifted. I think that recognizing the kids' VSL strengths might give these worried parents a little more confidence.
I have three late talkers so far, at least one of whom is gifted. I think one of the other ones, who had no words till 3 y.o., and is still in speech therapy at 6 y.o. for articulation issues, is likely to be the brightest of the three in the long run, but I guess only time will tell. Talk about a late bloomer...
