And Mark is a voice actor. A very good one. I believe the voice is his, but they have adjusted it to sound younger. I found his appearance in the episode to be very well done.
They used a program called Respeecher, a neural network powered voice synthesizer. Here’s Matt Wood on the process:
Matt Wood said:
It’s a neural network you feed information into and it learns. So I had archival material from Mark in that era. We had clean recorded ADR from the original films, a book on tape he’d done from those eras, and then also Star Wars radio plays he had done back in that time. I was able to get clean recordings of that, feed it into the system, and they were able to slice it up and feed their neural network to learn this data.
Isn’t cinema magical?