https://www.johndcook.com/powershell.html#:~:text=The%20core...
POSIX Korn shell, specifically, according to Wikipedia:
https://en.wikipedia.org/wiki/PowerShell#Grammar
so maybe it inherited 2>&1 from Korn shell, which in turn was POSIX.
But yeah, Powershell was not built purely to be a POSIX shell, but I thought it tried to be compatible where it made sense (hence the seeming clash of cultures).