Huh. The reply link appeared.

Anyway, yeah, there's a commented line mentioning that:

    /* Note true(1) will return EXIT_FAILURE in the
       edge case where writes fail with GNU specific options.  */
    atexit (close_stdout);

Makes sense. If you:

    $ /bin/true --version >& -

it fails because it was not able to write what you asked it to stdout.