Le is used here because the antecedent is a verb. Not using it is possible and likely the most frequent usage:

— Tu as dit à Paul de venir ?

— Oui, je le lui ai dit.

— Oui, je lui ai dit.

En would have been used in reply to that question where the antecedent is a noun:

— Tu as parlé à Paul de sa venue ?

— Oui, je lui en ai parlé.