According to the Twitter post by the manga’s editor Shintaro Kawakubo, Isayama turned down the offers from the program three times in the past, but finally decided to accept it with the thought of supporting the promotion for the TV anime third season.
The 30-minute episode follows his struggling days to create storyboards for the manga’s approaching climax.
Isayama says on the site, “I still don’t know why it has been sold so well.”