Maybe he reinserted the magazine after clearing the chamber, but before releasing the slide.
If Lou Horuchi and Rick Brana still have jobs, this guy has nothing to worry about.
That would be my guess. If the other parts of the article are accurate (not a safe assumption, but...) then that's the only thing that makes sense. He either put the magazine back in out of habit (he's an armed officer, loading his gun before holstering it would be his normal procedure), or the article might have missed the fact that the leg-shooting occurred *after* the "how to unload a gun" show-and-tell had concluded, and the officer was in the process of putting his sidearm away (after, of course, reloading it to return it to "ready" state).