If you wanted, you configure any computer, running any OS and connected to the internet, to be vulnerable to being used by spammers. Just because thousands (apparently) of OS9 users have left ports open to the net doesn't mean Macs are vulnerable.
It means the people who are responsible for configuring these systems aren't doing their job, and it doesn't help that the 3rd party proxy server they installed is configured by the developer to be vulnerable by default.
Macs be configured to be vulnerable, just like any network-savvy computer could be. From the article:
" ... You would think that proxy servers are handy for enforcing security, and in fact, they can be, if configured and deployed properly by a competent network administrator. ..."
You have to go out of you way to make a Mac open to remote vulnerabilties, and 4D shipped SW that made a very secure OS (OS9) into an insecure server simply by installing their product.
Remember, this is the same OS that one company was running when they published the IP address, and then challenged hackers to break in and read an unencrypted plain text file left on the HD, (they gave them the name of the file, too) while offering a huge cash prize to anyone who could tell them what the file said. Nobody won.