It walks through how some Starline Bass casings were loaded with live ammo, and then ended up on a film set where there should not have been any live ammo at all.
As to the armorer, yes, she was incompetent. That's the whole point here. The hiring director (who was not Baldwin) took a chance on someone who had past safety issues on her only other film, because she was the daughter of a well respected armorer.
She didn't know how to check the bullets to see if they were dummy rounds (completely fake, but realistic looking) or live rounds.
I know the article says blanks, but from everything I've found online, there weren't even blanks on the set of Rust. Just dummy rounds, and a few live rounds that snuck in via a coffee can full of co-mingled rounds from a previous film.