It's not an annulment; it's simply a defect of form that can be resolved by submitting a request to the diocese. Annulments involve apparently valid marriages that are determined not to be valid.
Legal marriages are considered apparantly valid by the Church. The Church will annul legal marriages if: 1) they have been legally terminated; and 2) (in short) the parties lacked the intent and capacity to marry. Annulments due to lack of form (or, if you will, defect of form) are no different from other annulments, they are just easier and quicker to prove. The tribunal is still ruling on the the question of whether the parties were not validly married, it's just that all that's needed is proof that one of the parties was Catholic, and that the "marriage" occured outside the Church without any dispensation. If so, the Catholic did not intend a sacramental marriage, and is therefore in the same boat as divorced Catholics whose previous marriages were invalid because they were found to be immature, cousins, forced to marry, etc.