Got to the Wikipedia page https://en.wikipedia.org/wiki/Slug_(publishing) which could possibly support the slimy conclusion of "it's a trail through the process" but that article has an etymology section that refers to the metal slug

I guess it could mean both depending on whether you're looking for the meaning of the word or the meaning of the concept but I didn't find any other slimy grub references (via an admittedly limited double check)