Yes, he is
Groot is him. The voice of Groot in all Avengers and Guardians of the Galaxy films is given by Vin Diesel. His range as an actor is incredible, and he proved that by being this character’s voice. Here to remind you that the voice of the lovable piece of wood named Groot is Vin Diesel.
He is proud of this role, and being proud of something he has done is nothing wrong. He is proud of himself for this accomplishment.