You mean the same Phoenix who died of food poisoning? Also, Roshi never said he got eternal life from the Phoenix, so that is speculation on your part. The cyborgs are also not immortal. Toriyama confirmed that they do aged, just slowly.Lionel wrote: The creature in which he acquired his immortality from was referred to as the Immortal Phoenix. We don't know when specifically the phoenix granted him eternal life because it's never made clear, though he did mistakenly try to give it to Goku and had to be reminded that it was gone by Turtle. Our only other confirmed instances of immortality are the cyborgs whom we know have eternal youth as part of the package because #18 was never depicted to have aged over the years.
I think we need to keep in mind that this statement is coming from a myopic point of view. Zamasu perceives himself to be above the machinations of humanity. He truly thinks of himself as being divine, and when that belief system is coupled with the immortality granted by a powerful wish fulfilling dragon, the notion of his self-ability is bound to increase as a result. His immortality has been contradicted by Trunks impaling him, though. It's not the equivalent of indestructibility, otherwise the blade would have held no impact and Zamasu wouldn't have a reason to avoid any of his opponent's attacks in the first place if they can cause literally no damage to him.
By all accounts, Zamasu is an immortal.


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