If I remember correctly he also reported a long while back that Matt Smith played a young Palpatine -post rejuvination- that was killed by Kylo but than his spirit possessed him and Kylo asked Rey to Kill him so that his spirit would die forever.

EDIT: found the link https://makingstarwars.net/2019/04/huge-rumor-about-how-the-final-duel-in-star-wars-the-rise-of-skywalker-plays-out/

-The rumor is that Matt Smith isn’t Palpatine reborn or rejuvenated but a dark side acolyte.
-However, during The Rise of Skywalker, Smith’s character is willfully possessed by the spirit of Emperor Palpatine.
-During the final act of the film Rey and Kylo team up to fight the Palpatine possessed Matt Smith.
-The two-on-one duel is pretty intense for the duo.
-Eventually Kylo Ren kills Palpatine’s host body
-Palpatine’s spirit then jumps into Kylo Ren’s body.
-Kylo Ren is strong enough to not lose total control of his mental faculties.
-Kylo Ren struggles to take control of his body but subdues himself and his weapon while beckoning Rey to kill him.
-Kylo does the right thing and becomes Ben Solo again as he sacrifices himself to kill Palpatine.
-Rey having never toyed with the dark side like Kylo Ren and the acolyte did means Palpatine has body evil enough to
inhabit.