Creating a reasonably secure compute environment requires elements from just about every area of system administration. Perhaps that’s why the LPI put security at the very end of their exam objectives, because you will, in fact, need all your skills to make this work.
I used the term “reasonably secure” because, when it comes to security, anyone who thinks his resources are 100% safe is fooling himself. An old friend who had worked for a national foreign service once told me that every single one of that country’s overseas embassies was provided with a government-issue hammer ...