It was not initially seen by the ref. Although as you say, cameras picked it up clear as day. The 4th official saw the replay, and alerted the ref that Zidane had used a headbutt. The ref was dealing with the scuffle afterwards and could well have just dished out yellows. But then FIFA would...
Zidane was sent off in the WC Final because of a video replay seen by the 4th official. In the years since FIFA have still claimed video evidence will slow the game. Idiots. I don't see a reason why the added time cannot carry on counting. It does on TV!! Again...idiots, all of em!