I'm not positive but I think that number includes lots of folks playing the campaign.
When I boot up Halo 2 it prompts me to sign in with my code before I can get to the menu to choose to play the campaign. Maybe this is because I do have Live!
When I am checking my friends list, I can see whether they are in the campaign or in multiplayer. So I would bet that as many as 50% of that number (80,000 at peak times) is playing the campain!