I work with a consulting firm, and while I do mostly private sector projects, I get stuck in DoD and DHS projects sometimes....
The DHS is a tremendously broken organization. It is bloated, wasteful, filled with overpaid middle management idiots, run by overpaid middle management idiots, and has a conservative (in a bad way) culture due to the politically biased security clearance process of the United States. (What I mean by that is the security clearance process weeds out left of center people. Peace Corps, attended school at Berkeley, smoked pot? Forget about it, you're out. Well, the smartest people often fit this profile. And they ain't at DHS)
This article hits the nail on the head in one sense. The hiring process is broken for the Federal government, but so is the firing process. If you are a productive individual and you work in a place where lazy, late to arrive, early to leave, incompetent individuals are not only tolerated but promoted, why would you stay? If you do stay, why would you work hard? That's the US Federal gov't GS workforce, particularly the DoD and DHS, in a nutshell.
TLDR: The Federal government is not only slow to hire, but because it doesn't fire, let alone identify incompetent and lazy shitbags, the Dead Sea effect is in constant overdrive with competent people.