Personally I always felt that the film makes a clear implication that the power comes from the bond of the dyad.

The lifeforce of your bond… a dyad in the Force. A power like life itself. Unseen for generations. And now… the power of two restores the one, true Emperor.

(and then he drains their energy - nearly killing them - and restores his damaged body)

The only thing that confuses this is that Grogu has the power in the Mandalorian, but then you’re dealing with stuff outside the movie.