In conjunction with the cloning tech witnessed when Kylo Ren first met Palpatine, it was a mystery that was explained at the climax of the movie with the surprise twist and revelation that the Sith transfer their spirits and powers into the apprentice's body when apprentice kills master, something that had not happened with Palpatine, because Luke had refused to do in ROTJ and because Vader did not do it either, or didn't do in the "right way," because he'd simply chucked Palps down a very deep shaft into the main reactor.
Also, it was pretty clearly implied in ROTJ that Palpatine's spirit survived the disintegration of his body, because his death cry continued well after his vocal cords had been incinerated, while a weird blueish glow whooshed back up out of the reactor chasm. Strange how people didn't consider that important.
Poe's clunky line was neither the beginning nor the end of the film's disclosures about how the Emperor had come back from the dead.
I mean...
Yeah, this. It had plenty of both, more than enough to make it clear.
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edited to add:
We've had Force ghosts since Obi-Wan shouted, "Run, Luke! Run!" and visible Force ghosts since Obi-Wan told Luke to seek Yoda in the Dagobah system. Was nobody wondering what Palpatine's Force ghost was up to, or if he even had one too?
Luke's Force ghost even managed to lift his own X-wing out of the sea, something else TROS did to help explain Palpatine's return by establishing that even Force ghosts can interact consequentially with the material world....