You should be able to copy&paste that to your console/SSH client, although sometimes tab characters (contained in the lighttpd.conf) create issues. In that case edit /etc/lighttpd/lighttpd.conf manually and copy/replace with the part from server.modules til above _EOF_ (the "here document").
G_CONFIG_INJECT are commands in DietPi bash shells, yes. They alter or add settings or uncomment commented settings, depending on if/how they are present. I wanted a generic way to "add a setting, if it's missing, else, if it is present, but commented, uncomment and set it, and if it is present and active, alter it, print to console what has been done (and respect a few options that can be given, like masking passwords).", especially for our scripts, so created this.