I don't think that question was closed by the community for being "below the bar" of knowledge, but rather the question was poorly phrased, and seemed that the asker did not even give thought to what he was asking.
I don't think we need to have a "minimum expected knowledge" at all - when it comes to security.
A complete security newbie is welcome to ask introductory level questions about security.
However, I think it is fair to expect that the asker at least have a basic knowledge about the content of his question, i.e. whatever it is he's asking security about.
If you're asking the most basic of questions on how to secure an Apache webserver - that's fine, if you know what an Apache webserver is (and how to configure it).
If you're asking how to prevent SQL Injection in PHP, from the ground up - that's great, if you know how to write code in PHP.
If you're asking how to bypass a firewall - you should probably know what a firewall is and what it does (though asking about that here is also okay, but that is a completely different question). You should probably have a clue about how networks work, how they're set up, the difference between a network and a workstation, and so forth.
In other words: we should be willing to educate even kindergarden level - about security. Nothing else. (And btw, even high-level non-security questions would be offtopic, anyway.)
Note, however, that sometimes the simplest questions are often answered on Wikipedia, and as always we do expect the asker to do at least a minimal amount of research, before asking.
I think the reason the linked question was closed, was because it was assuming a level of (non-security) knowledge, that was not apparently there. It felt to me that the asker didn't understand network basics, or really what a firewall is and does (and does not do). Or perhaps the question just left a large gap in specificity.
I think that had the asker just came out and said "WTF is this firewall thing, and what does it do for my network?" it would have been ontopic, though then answers would probably have been best served by RTFWA (read the Wikipedia article).