It looks like my suspicion that "mate-power-backlight-helper" is referred to by absolute
file name instead of by its polkit action is correct:
Excerpt from src/gpm-brightness.c
(https://git.mate-desktop.org/mate-power-manager/tree/src/gpm-brightness.c)

> static gboolean
> gpm_brightness_helper_set_value (const gchar *argument, gint value)
> {
>         [...]
>         /* get the data */
>         command = g_strdup_printf ("pkexec " SBINDIR "/mate-power-backlight-helper --%s %i", argument, value);
>         ret = g_spawn_command_line_sync (command, NULL, NULL, &exit_status, &error);
>         [...]
> }

Next step: look at the GNOME source code, maybe it does things differently.