He isn't generally sloppy about that sort of thing, so he may well be right, but I've heard Cherryh and Bujold talk about what it took to go full-time, and it would surprise me if there were as many as two hundred in the history of the field. Subtract anyone who's living on inherited money (was their anyone other than Niven?) and I suppose we should put an asterix by the name of anyone who died of poverty.
For purposes of this discussion, it might be reasonable to include people who make a substantial part of their money from sf tie-ins, but I think they should be selling at least some original work.
Also, is it harder to go full-time now than it used to be?