To be clear:
The 8080 siteadmin is the legacy site admin, you use your FTP password, as it's written in perl it has some ugly quirks/hacks in it.
the siteadmin site itself you use your forum username, you need to tie your ftp login to it to use it, it was designed around having multiple comics without having to login to every comic on the system to trigger updates and so forth.
The quickadmin uses the ftp login and is used primarily to bypass the siteadmin without having a forum.
The problem overall is that I've had to remove dozens of sites over the years due to spambots taking over comics, presumably grabbing the ftp password from emails of compromised free email accounts.