Admin error or not is not the point. Most Windows malware is also caused by admin error or oversight (i.e. not patching, or running unsecure versions of plugins and software; not having a decent firewall, etc). People tend to forget that security is never 100% unless you know what you're doing. There is no cure all, simply running Linux will not protect you
from vulnerabilities if you do not secure your OS.
The point of my posting this here was to make sure that fact was known. No matter if you choose to use OS X, Windows or any of the different flavours of Linux, you still have to be proactive about security. In this case, the admins were careless, the result is evident.
Not a botnet? I beg to differ. It's a collection of Linux servers that fell in to the control of someone else (which is part of the definition of a bot
); there were many discovered, thus a network of bots. Whether they use it to DDoS or to host malicious files and infect other computers, has no bearing on that fact. Whether it's used to spread malware aimed at Windows machines as opposed to *nix ones, the result stands.