I'm not, to my knowledge, using cygwin or anything like it.

But you do have Cygwin or something like it (maybe msys ?). Nothing else is going to report a path that begins with "/usr/bin".

I have both Cygwin and Msys:

C:\>which which
'which' is not recognized as an internal or external command,
operable program or batch file.

C:\>set PATH=C:\_64\msys64\usr\bin;%PATH%

C:\>which which
/usr/bin/which

C:\>
[download]

and, for Cygwin:

C:\>which which
'which' is not recognized as an internal or external command,
operable program or batch file.

C:\>set PATH=C:\cygwin\bin;%PATH%

C:\>which which
/usr/bin/which

C:\>
[download]

Cheers,
Rob