That's a lie - YES IT DOES.
It does not.
If united here means "combined with" it makes no sense. Is it alleged was she a goddess until Christ's death? Of course not; your suggestion produces a nonsensical result.
However, since united here means "harmonious mutual action, cooperation" it makes perfect sense. In reading CCC 964, it becomes abundantly clear that "combined with" is not what is meant by the text.