Whats the difference between a siphonophore and a single organism? Aren't all the organs of an organism genetically identical, specialized, fused together, and functionally dependent on each other?

It's a very fuzzy line. But according to The Octopus Lady's video in the other comment, it's because separating them from the other zooids doesn't result in immediate death. They may die later due to lack of ability to swim, or eat, but that is a secondary cause which is considered important.