To be fair, it only works for jellyfish that are a few millimeters from tip to toe, so to speak. And it's never been tested in the wild with any success. And it doesn't stand up to diseases. But apart from that, total immortality!
Personally, I think the Yew tree is closer to immortality. When they get a usually deadly disease, they just turn one branch in on themselves, turn it into a root and use it to replace their trunk completely. I mean, still vulnerable to predators, but they're still way ahead of jellyfish.