Sinai traditions, Moses' radiant face, Elijah at Horeb, and Deuteronomy's prophet-like-Moses expectation all contribute to the scene.
Moses and Elijah appear in glory, but the Father's voice singles Jesus out, and when the revelation ends Jesus is found alone.
Created figures may reflect glory, so radiance alone is not proof. The complete theophanic scene and the Father's unique testimony make this strong evidence.
The disciples are afraid and initially keep silent.