Most blanket statements of that nature are hyperbole, if not intentional falsehood. I find that it's generally wise to avoid them.
And I would be more impressed by a 50/50 chance if Exodus' definition of "success" were a little less broad. Helping Christians to be celibate instead of gay -- if that's what they want -- is certainly better than pushing them into heterosexual relationships. But it doesn't add much weight to the argument that orientation is subject to change.