ksh is a superset of Bourne shell https://www.oilshell.org/archive/ksh-usenix.pdf https://www.oilshell.org/archive/ksh-usenix.pdf bash implemented essentially

ksh is a superset of Bourne shell https://www.oilshell.org/archive/ksh-usenix.pdf https://www.oilshell.org/archive/ksh-usenix.pdf bash implemented essentially all Bourne shell features a long time ago -- and for the last decade+, and currently, it's implementing ksh features I'm not sure why it's hard to hold those 2 ideas in your head at the same time