How was Shinji Hirako listening to 1940/1950s jazz music over 100 years ago?
Jazz originated at the beginning of the 20th century and incorporated music from 19th and 20th century, but you would have to ask Kubo why he drew that as we do not really know and he has not revealed anything about it. The music that played in the anime is the anime studio’s choice, which is not necessarily either accurate or what Kubo intended.
