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.