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).
I don't think they were talking about pwsh? pwsh actually has types and is its own programming lang unlike *sh, so it doesn't rely on builtin command exit codes.