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.
It's just a fact about the evolutionary history. You are a "multicellular" organism; you are composed of a bunch of genetically identical cells that fuse with each other and differentiate their development to specialize in certain tasks. It's not just your organs; your entire being is like that.
Siphonophores are identical, except that they can be subdivided into units that are, much like organs, composed of multiple cells. Those units can be identified as evolutionary derivations of certain branches of the tree of life. This is more similar to your mitochondria, which are (like zooids) identifiable as having once been independent organisms, though unlike zooids your mitochondria are not genetically identical to you.
True. And also amazing how complex their interactions can be! Just because immune cells are often individually motile, does not mean they're not intimately interoperating within an incredibly complex social ecology of cells. https://en.wikipedia.org/wiki/T_cell is full of surprises.
