jobserver fds remain open in subprocess

[Keith Rarick]

I'm seeing some behavior that surprises me:
https://gist.github.com/b4852e149f45a43fb17a

I'd expect that putting a + in front of the command
would cause make to leave the jobserver fds open,
and leaving off the + would cause make to close
those fds. This is the behavior described in
http://mad-scientist.net/make/jobserver.html and
it matches my reading of the make source code
(retrieved from http://ftp.gnu.org/gnu/make/make-3.82.tar.gz).

Can anyone help me understand what's going on?