Standardization does not work like this. Usually the consideration is that the standard standardizes the greatest common subset, subject to some give and take, of available implementations, which at that time included more than the Korn shell.
There's an entire rationale that accompanies the the Single Unix Specification, whose section on the shell command language quite clearly explains the basis for the standard. Talking about how POSIX based things on the Korn shell is to not even have read the very first sentence of that rationale section.
I clearly wrote "the very first sentence of that rationale section". Even though you've erroneously picked the standard from 2004 instead of the current edition, and then erroneously didn't even look at the rationale, the first sentence of that section of that old edition's rationale is the same.
There's an entire rationale that accompanies the the Single Unix Specification, whose section on the shell command language quite clearly explains the basis for the standard. Talking about how POSIX based things on the Korn shell is to not even have read the very first sentence of that rationale section.