This was a vision , we know that Moses and Elijah had died , but this vision represented
more than just a vision, but he was facing his death soon to come by his enemies.
During that event Notice what also took place
Mt 17:2 And he was transfigured before them; his face shone as the sun, and his outer garments became brilliant as the light.
, Mt 17:5 While he was still speaking, look! a bright cloud overshadowed them, and look! a voice out of the cloud said: “This is my Son, the beloved, whom I have approved. Listen to him.”
The transfiguration,or vision , it seems, served to fortify Christ for his sufferings and death, while it also comforted his followers and strengthened their faith. It showed that Jesus had God’s approval, and it was a foreview of his future glory and Kingdom power. It presaged the presence of Christ, when his kingly authority would be complete.
Also The three that witnessed this vision were told not to report it at that time only after Jesus resurrection.
We can say that that was a ancient hologram , but
We know it is not real.