Its my understanding that everyone discussed the plot and agreed to the idea. But not to doing the movie. They all just like the concept.
Now that concept is being put into script. The only people in the know about the script itself are Aykroyd, Ramis, and the writers. Once thats finished and they like what they have, they'll goto Bill and Ernie and get them on board too.
I like that Bill is still taking this attitude, because it forces the writers to really focus on the story, and tbh, having recently gotten into The Office reruns, I am more optimistic then ever, they are stepping in the right direction.
But anyone who thinks this is already a total go, is nuts.