An opening statement isn't evidence, so really the defense can say whatever they want. Scott Peterson's attorney did the same thing, made all kinds of grandiose statements and then (iirc) rested his case after the prosecution was done.
Even if she never takes the stand, the defense's statements are still out there, and if it sways one juror, they've done their job.
Casey's attorney would be an idiot to put her on the stand because then the prosecution gets a chance to skewer her. But from what I've heard, he IS an idiot, and Casey thinks she such a good liar, so who knows.