That description could match tens of thousands of other women in California, as could the description of the car. In itself it is coincidental, not definitive. It would be used as corroboration for other more unambigous evidence, such as Jaycee's direct testimony for example. Alternatively, if physical evidence was left at the scene that could match either Nancy or the car to the crime forensically, that could be used in place of Jaycee (or Nancy or PG's) testimony to link the Garrido's to the abduction, but as far as we know there isnt any such evidence. That leaves testimony from one of the principals to prove the kidnapping charge.
It would be better for the prosecution if they had shown Carl a line-up before Nancy's picture was published by the press, but as far as we know, that didn't happen. If they had done that, then his identification would carry weight. If they didn't, then it could be argued that he was identifying her after her picture was made publically known, in which case it would be useless, since he would be identifying the woman he saw on TV, not the woman in the car.
The other potential issue with Carl, as I pointed out earlier, is that he has recounted his story god knows how many times in public over the years, and unless it is 100% accurate in all those retellings the defense lawyers are going to use that to cast doubt on the accuracy of what he does eventually say in court. The moral of the story is that in these sorts of situations where you one day may be asked to testify on an incident that is very personal to you, tell it to LE when required to do so, but beyond that keep your mouth shut.
