You need to critique what they actually said, not what you make up out of whole cloth.
DISCRETIONARY spending includes everything that is not specifically required by law. It has nothing to do with how "necessary" the expenses in question are, only with how much control can be immediately exercised over them.
The proper response to an increase in necessary spending is to make corresponding cuts in non-necessary spending. The ongoing and blatant failure to do so is, quite rightly, criticized by Cato.
Take a break guy, it was someone else who said "uh..homeland security is not an unnecessary expense" or something like that and I commented on what he said. Now, was he wrong? Could be. Did he "make it up out of whole cloth"? I don't know but I doubt it. Even people who are wrong on an internet web site are not necessarily malevolent.