Nah, the Halo 2 release date was already pretty much fixed.
And it wasn't like the source code was stolen. That would have been a serious problem. But in this case it was a compiled build. Considerably less security issues, though it gives the ISO crackers a leadtime on preparing for the English language release.
In the case of the Half Life 2 leak, it was source code and therefore subject to examination and easy exploitation. So they had no choice but to delay.
The version of Halo 2 out there now is thought to have been stolen from the pre-pressing scraps at a manufacturing plant. A lot of pre-retail leaks happen that way.
Plus there's the rumors of certain video card companies leaking games, but I won't name any names
If someone wants to steal themselves a copy of Halo, or any of the games we make here, then that sucks, but oh well. If they then brag about having it, they should be very careful who they brag in front of. Not all game dev's are 98 lbs pimple farms