star wars - Why didn't Palpatine do what he threatened to do?

At the end of Star Wars: The Rise Of Skywalker, Palpatine tells Rey...

...that if she kills him, he and all the Sith will possess her.

Rey decides...

...not to kill Palpatine. But then decides to kill him about 10 minutes later using the power of all the Jedi.

So then why didn't he possess her after all?