The immortality itself doesn't grant it. I'm basically looking at this like, a story is a page in the book and Epilogue is the phenomenon that closes it. The end of a story is the end of the existence itself so "closing" the stories basically means closing the book, Epilogue isn't exactly the book, it's the phenomenon that closes it and to close the book you need to be outside of it.
I do get where you're coming from tho, it certainly seems to be just R>F difference tho imo it can be both, since we know there's nothingness before the story's beginning, why can't there be nothingness after it's end, especially when we're told that existence itself ends when a story ends. But yea, comparing Prologue and Epilogue feels wierd but.... just makes sense? Honestly, this thing even has a bit of weight just cuz Prologue came from Epilogue.
We know FP Prologue can affect Omnipotent and Omniscient beings and UE Kuro was able to damage Makina's main body so there's that. Prologue was also stated to be THE strongest after Epilogue but that was way back in the past tbh but the argument can still be made exactly cuz we dunno the full power of Prologue.