I don't think so. When I say the future was decided at that moment, I mean that in the sense that cause logically followed from effect, right from the beginning until the end, so that the future of the universe can be derived right back to the beginning of the universe (actually not sure if this is true, chance might play a considerable role). However, amongst those many causes that determine the future, there is still such factors as Truman's own preferences, his own "will". In other words, he still made his own choice, even though it was decided right at the very beginning of the universe what that choice was going to be.
I hope that makes it any clearer, though I admit it doesn't seem that clear even to me.