They also say that production is 500k. That isn’t true (yet), but they didn’t make the number up. I think they future proofed their documentary (which will be broadcasted for a couple of years, I guess) by mentioning that number. Similarly, the 13 million may have been a future number they picked up when filming.Exactly because that video is sloppy, I don’t expect them to came up with the number by doing research, using total capacity/cell capacity etc.
So I suspect they heard the number from some line workers or low level PR/managers, and used it as is without a second thought.