They're not even technically one organism, but colonies of independent but mostly specialized organisms. I'd be willing to bet that has something to do with the articles title
True jellyfish (like moon jellies, box jellyfish)are a single organism, just like you or me. Theres a single genome and one body.
Portuguese man o’ war is not a single organism at all but a siphonophore, a colony of many genetically identical but specialized individual organisms called zooids, all fused together and functionally dependent on each other.
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?