It's my understanding that certain turtles don't exhibit telomere shortening, and are effectively 'immortal' as well in that they don't age.
Do they not get cancer?
I think that neither really makes them less 'immortal' in the sense that this article means.
Though I can see the argument that errors in cell replication leading to cancerous growths could be counted as a form of natural life span limitation, I would be surprised if it didn't also happen to this jellyfish.