The addressee here could be his own people where calling Allah Ta’ ala their Lord was to express a reality - though, they did not accept it. Then, it could also be that this address is to the messengers and the purpose of saying: إِنِّي آمَنتُ بِرَبِّكُمْ فَاسْمَعُونِ (listen to me) may be that they should hear what he was saying and bear witness before Allah that he was a believer.