I probably output the required css to html directly. If I remember correctly it is a one liner...
Took a brief look in the code and it doesn't look to me like you are outputting it at all. Any Process module automatically gets any CSS or JS files with the same name included, so it's ProcessWire that's doing it, not ProcessRedirects. ProcessWire does that because it assumes Process modules aren't autoload. It's unusual for a Process module to be autoload, though it makes perfect sense with ProcessRedirects. So the best solution is to just work around it and use a different name for the CSS file (like styles.css) and add it manually, i.e.
$config->scripts->add($config->urls->ProcessRedirects . "styles.css");
Not an urgent thing by any means. I will submit a pull if the need comes up here before you get to it.