Wait, so people think "IT" means desktop support? Network architect is IT. Sysadmins, network admins, "devops" (remember when they were called sysadmins?) are all IT.
IT is about the technology. IS (information systems) is about the system as a whole, which includes the people. IT is focused on making technology work, IS is focused on using technology to improve a process.
IT is a part of IS, but IS is bigger than IT. If your job focuses on fixing technology (such as computer repair), you're working IT. If your job focuses on processes and people (such as software development, information security, business enablement, etc), you're working in IS.
For your website, the IT portion is the server it runs on. The IS portion is the IDE you use, your development environment, the server, your language/framework, the browser, your developers, and your customers. That's the information system as a whole that allows your website to work.
But yeah, when I hear "IT guy", I think desktop support. People in IS outside of support tends to refer to themselves as something more specific than general IT: I'm a programmer/developer/software engineer. I'm a network administrator. I'm a systems architect. I'm a security engineer. That's just my experience, and that's what I interpreted the comment "stay out of IT" to mean. Desktop support doesn't tend to have much room for growth compared to careers in IS.
Language is a reflection of the idea being presented. If your audience perceives IT to be desktop support, then using IT to mean desktop support is absolutely correct and does reflect reality. This is my interpretation of tptacek's comment, and from his posts afterwards it seems I am right. Other comments here are saying the same thing, that IT is a "computer janitor".
This might be different in other English-speaking markets, but at least in this thread, that is what is being expressed. It doesn't matter what term is being used, that is what the parent was trying to get across. Don't become "the IT guy". The definition of IT guy he was using is "computer janitor".
"IT" is now being used more often to mean "computer janitor" than all the jobs related to computer/system operations. It's a bit of a shift in the terminology.