I interpreted that the other way around.
Leia becomes one with Ben (sort of force projects into him), mirroring Luke’s sacrifice in TLJ.This explains why he’s able to fight the Knights of Ren, survive the death pit and transfer life into Rey (*).
It’s Ben’s + Leia (as a full jedi master) combined powers. So when Ben dies, they both “rise” at the same time, since they are joined by the force.(*) While Rey is dead, Ben is the last living Jedi.
That’s pretty much what I got, wasn’t sure how to articulate that well enough.